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Biochemical and functional characterization of the interaction between liprin-α1 and GIT1: implications for the regulation of cell motility.

Asperti C, Astro V, Pettinato E, Paris S, Bachi A, de Curtis I - PLoS ONE (2011)

Bottom Line: Here we have addressed the involvement of the liprin-α1 partner GIT1 in liprin-α1-mediated effects on cell spreading and migration.No cumulative effects were observed after depletion of both liprin-α1 and GIT1, suggesting that the two proteins belong to the same signaling network in the regulation of cell spreading.These findings and the possible competition of liprin-α1 with paxillin for binding to GIT1 suggest that alternative binding of GIT1 to either liprin-α1 or paxillin plays distinct roles in different phases of the protrusive activity in the cell.

View Article: PubMed Central - PubMed

Affiliation: Division of Neuroscience, San Raffaele Scientific Institute and San Raffaele University, Milano, Italy.

ABSTRACT
We have previously identified the scaffold protein liprin-α1 as an important regulator of integrin-mediated cell motility and tumor cell invasion. Liprin-α1 may interact with different proteins, and the functional significance of these interactions in the regulation of cell motility is poorly known. Here we have addressed the involvement of the liprin-α1 partner GIT1 in liprin-α1-mediated effects on cell spreading and migration. GIT1 depletion inhibited spreading by affecting the lamellipodia, and prevented liprin-α1-enhanced spreading. Conversely inhibition of the formation of the liprin-α1-GIT complex by expression of liprin-ΔCC3 could still enhance spreading, although to a lesser extent compared to full length liprin-α1. No cumulative effects were observed after depletion of both liprin-α1 and GIT1, suggesting that the two proteins belong to the same signaling network in the regulation of cell spreading. Our data suggest that liprin-α1 may compete with paxillin for binding to GIT1, while binding of βPIX to GIT1 was unaffected by the presence of liprin-α1. Interestingly, GIT and liprin-α1 reciprocally regulated their subcellular localization, since liprin-α1 overexpression, but not the GIT binding-defective liprin-ΔCC3 mutant, affected the localization of endogenous GIT at peripheral and mature central focal adhesions, while the expression of a truncated, active form of GIT1 enhanced the localization of endogenous liprin-α1 at the edge of spreading cells. Moreover, GIT1 was required for liprin-α1-enhanced haptotatic migration, although the direct interaction between liprin-α1 and GIT1 was not needed. Our findings show that the functional interaction between liprin-α1 and GIT1 cooperate in the regulation of integrin-dependent cell spreading and motility on extracellular matrix. These findings and the possible competition of liprin-α1 with paxillin for binding to GIT1 suggest that alternative binding of GIT1 to either liprin-α1 or paxillin plays distinct roles in different phases of the protrusive activity in the cell.

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Binding of liprin-α1 to GIT1-C2 prevents binding of paxillin to GIT1-C2.(A) Lysates were prepared from COS7 cells transfected with either HA-GIT1-C2 (C2) or co-transfected with HA-GIT1-C2 and FLAG-liprin-α1 (C2+Lip). Aliquots of the lysates were used for immunoprecipitation with anti-paxillin antibodies (IP anti-paxillin, 400 µg of protein per IP). Filters with immunoprecipitates (a), and with 100 µg of both lysates (Lys) and unbound fractions after IP (Ub) (b) were cut and immunoblotted with anti-Flag to detect Flag-liprin-α1 (upper filters, only one of the duplicated immunoprecipitations is shown); since GIT1-C2 and paxillin migrate at similar positions on gels, the lower parts of the filters from the duplicated immunoprecipitations were used as follows: one set of filters (a+b) was incubated with anti-HA to detect HA-GIT1-C2 (middle blots), and one set was incubated with anti-paxillin to detect endogenous paxillin (lower blots). Paxillin was absent from the unbound fractions after immunoprecipitation (Ub). (c) The unbound fraction (300 µg) after immunoprecipitation with anti-paxillin from the lysate of cells co-transfected with HA-GIT1-C2 and FLAG-liprin-α1 [Ub(C2+Lip)], was re-immunoprecipitated with anti-liprin antibody, to reveal the presence of the liprin-α1/GIT1-C2 complex in the lysate. (B) Binding of liprin-α1 to GIT1-C2 does not prevent binding of βPIX to GIT1-C2. Identification of a ternary complex among liprin-α1, βPIX and GIT1-C2. COS7 cells co-transfected to express the indicated combinations of HA-GIT1-C2, HA-βPIX, and FLAG-liprin-α1 were immunoprecipitated with anti-FLAG antibodies (top blots on the left). Aliquots of the unbound fraction after the first round of immunoprecipitations were re-immunoprecipitated with anti-βPIX antibodies (top blots on the right). Filters including immunoprecipitations (IP), lysates (Lys), and unbound fractions after the second round of immunoprecipitations (Ub) were cut and blotted as indicated (lower blots). (C) Liprin-α1 does not interfere with the interaction of βPIX with GIT-C2. COS7 cells co-transfected to express the indicated combinations of HA-GIT1-C2, HA-βPIX, and FLAG-liprin-α1 were immunoprecipitated with anti-βPIX antibodies. Filters including aliquots of lysates and the immunoprecipitations (IP) were cut and blotted as indicated. (D) A COS7 cell lysate (1 mg protein) was immunoprecipitated with anti-βPIX antibodies. Immunoprecipitate (IP) and equal amounts (100 µg) of lysate (Lys) and unbound fraction (Ub) were blotted with anti-GIT (mAb PKL, recognizing both GIT1 and GIT2 proteins, on the left; or anti-GIT2-specific pAb, on the right), βPIX, or anti-liprin-α1 antibodies. Blot with anti-GIT antibody was performed after stripping the filter incubated for βPIX. (E) binding of βPIX to full length GIT1 does not enhance the binding of liprin-α1 to GIT1. COS7 cells were co-transfected with FLAG-liprin-α1 and FLAG-GIT1, or with FLAG-liprin-α1 and FLAG-GIT1 and HA-βPIX. 200 µg of each lysate were immunoprecipitated with anti-GIT1 antiserum. Lysates (Lys, 50 µg), unbound fractions (Ub, 50 µg) and immunoprecipitates were blotted and incubated with antibodies specific for the indicated proteins. Overexpression of βPix did not increase the interaction of liprin-α1 with GIT1. (F) Model for the regulated interaction of GIT1 with paxillin and liprin-α1. Either ligand binds poorly to full length GIT1. We hypothesize that activation of GIT1 by so far unknown mechanisms is required for the formation of either GIT1/paxillin or GIT1/liprin-α1 complexes.
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pone-0020757-g001: Binding of liprin-α1 to GIT1-C2 prevents binding of paxillin to GIT1-C2.(A) Lysates were prepared from COS7 cells transfected with either HA-GIT1-C2 (C2) or co-transfected with HA-GIT1-C2 and FLAG-liprin-α1 (C2+Lip). Aliquots of the lysates were used for immunoprecipitation with anti-paxillin antibodies (IP anti-paxillin, 400 µg of protein per IP). Filters with immunoprecipitates (a), and with 100 µg of both lysates (Lys) and unbound fractions after IP (Ub) (b) were cut and immunoblotted with anti-Flag to detect Flag-liprin-α1 (upper filters, only one of the duplicated immunoprecipitations is shown); since GIT1-C2 and paxillin migrate at similar positions on gels, the lower parts of the filters from the duplicated immunoprecipitations were used as follows: one set of filters (a+b) was incubated with anti-HA to detect HA-GIT1-C2 (middle blots), and one set was incubated with anti-paxillin to detect endogenous paxillin (lower blots). Paxillin was absent from the unbound fractions after immunoprecipitation (Ub). (c) The unbound fraction (300 µg) after immunoprecipitation with anti-paxillin from the lysate of cells co-transfected with HA-GIT1-C2 and FLAG-liprin-α1 [Ub(C2+Lip)], was re-immunoprecipitated with anti-liprin antibody, to reveal the presence of the liprin-α1/GIT1-C2 complex in the lysate. (B) Binding of liprin-α1 to GIT1-C2 does not prevent binding of βPIX to GIT1-C2. Identification of a ternary complex among liprin-α1, βPIX and GIT1-C2. COS7 cells co-transfected to express the indicated combinations of HA-GIT1-C2, HA-βPIX, and FLAG-liprin-α1 were immunoprecipitated with anti-FLAG antibodies (top blots on the left). Aliquots of the unbound fraction after the first round of immunoprecipitations were re-immunoprecipitated with anti-βPIX antibodies (top blots on the right). Filters including immunoprecipitations (IP), lysates (Lys), and unbound fractions after the second round of immunoprecipitations (Ub) were cut and blotted as indicated (lower blots). (C) Liprin-α1 does not interfere with the interaction of βPIX with GIT-C2. COS7 cells co-transfected to express the indicated combinations of HA-GIT1-C2, HA-βPIX, and FLAG-liprin-α1 were immunoprecipitated with anti-βPIX antibodies. Filters including aliquots of lysates and the immunoprecipitations (IP) were cut and blotted as indicated. (D) A COS7 cell lysate (1 mg protein) was immunoprecipitated with anti-βPIX antibodies. Immunoprecipitate (IP) and equal amounts (100 µg) of lysate (Lys) and unbound fraction (Ub) were blotted with anti-GIT (mAb PKL, recognizing both GIT1 and GIT2 proteins, on the left; or anti-GIT2-specific pAb, on the right), βPIX, or anti-liprin-α1 antibodies. Blot with anti-GIT antibody was performed after stripping the filter incubated for βPIX. (E) binding of βPIX to full length GIT1 does not enhance the binding of liprin-α1 to GIT1. COS7 cells were co-transfected with FLAG-liprin-α1 and FLAG-GIT1, or with FLAG-liprin-α1 and FLAG-GIT1 and HA-βPIX. 200 µg of each lysate were immunoprecipitated with anti-GIT1 antiserum. Lysates (Lys, 50 µg), unbound fractions (Ub, 50 µg) and immunoprecipitates were blotted and incubated with antibodies specific for the indicated proteins. Overexpression of βPix did not increase the interaction of liprin-α1 with GIT1. (F) Model for the regulated interaction of GIT1 with paxillin and liprin-α1. Either ligand binds poorly to full length GIT1. We hypothesize that activation of GIT1 by so far unknown mechanisms is required for the formation of either GIT1/paxillin or GIT1/liprin-α1 complexes.

Mentions: Based on these findings, we tested the hypothesis that liprin-α1 may interfere with the binding of paxillin to the carboxy-terminus of GIT1 in the cell. For this, we first immunoprecipitated endogenous paxillin from lysates of cells transfected either with HA-GIT1-C2 alone, or with both HA-GIT1-C2 and full length FLAG-liprin-α1. Under conditions in which endogenous paxillin was virtually immunodepleted from lysates (Fig. 1, A, panels a and b), the interaction of paxillin with HA-GIT1-C2 was strongly reduced in the lysates from co-transfected cells (Fig. 1, A, panel a). We then tested the hypothesis that the decrease of binding of paxillin to GIT1-C2 may be due to binding of the overexpressed liprin-α1 to GIT1-C2 itself. For this, the unbound fraction after immunoprecipitation with anti-paxillin from lysates of cells co-transfected with HA-GIT1-C2 and FLAG-liprin-α1, was used in a second round of immunoprecipitation with anti-liprin-α1 antibody (Fig. 1, A, panel c). This immunoprecipitation showed a strong interaction of FLAG-liprin-α1 with HA-GIT1-C2 (Fig. 1, A, panel c). These data suggest that binding of overexpressed liprin-α1 to the carboxy-terminal portion of GIT1 interferes with the binding of paxillin to the same region of GIT1, and indicate that the formation of a trimeric liprin-α1/GIT1/paxillin complex in the cell is not likely.


Biochemical and functional characterization of the interaction between liprin-α1 and GIT1: implications for the regulation of cell motility.

Asperti C, Astro V, Pettinato E, Paris S, Bachi A, de Curtis I - PLoS ONE (2011)

Binding of liprin-α1 to GIT1-C2 prevents binding of paxillin to GIT1-C2.(A) Lysates were prepared from COS7 cells transfected with either HA-GIT1-C2 (C2) or co-transfected with HA-GIT1-C2 and FLAG-liprin-α1 (C2+Lip). Aliquots of the lysates were used for immunoprecipitation with anti-paxillin antibodies (IP anti-paxillin, 400 µg of protein per IP). Filters with immunoprecipitates (a), and with 100 µg of both lysates (Lys) and unbound fractions after IP (Ub) (b) were cut and immunoblotted with anti-Flag to detect Flag-liprin-α1 (upper filters, only one of the duplicated immunoprecipitations is shown); since GIT1-C2 and paxillin migrate at similar positions on gels, the lower parts of the filters from the duplicated immunoprecipitations were used as follows: one set of filters (a+b) was incubated with anti-HA to detect HA-GIT1-C2 (middle blots), and one set was incubated with anti-paxillin to detect endogenous paxillin (lower blots). Paxillin was absent from the unbound fractions after immunoprecipitation (Ub). (c) The unbound fraction (300 µg) after immunoprecipitation with anti-paxillin from the lysate of cells co-transfected with HA-GIT1-C2 and FLAG-liprin-α1 [Ub(C2+Lip)], was re-immunoprecipitated with anti-liprin antibody, to reveal the presence of the liprin-α1/GIT1-C2 complex in the lysate. (B) Binding of liprin-α1 to GIT1-C2 does not prevent binding of βPIX to GIT1-C2. Identification of a ternary complex among liprin-α1, βPIX and GIT1-C2. COS7 cells co-transfected to express the indicated combinations of HA-GIT1-C2, HA-βPIX, and FLAG-liprin-α1 were immunoprecipitated with anti-FLAG antibodies (top blots on the left). Aliquots of the unbound fraction after the first round of immunoprecipitations were re-immunoprecipitated with anti-βPIX antibodies (top blots on the right). Filters including immunoprecipitations (IP), lysates (Lys), and unbound fractions after the second round of immunoprecipitations (Ub) were cut and blotted as indicated (lower blots). (C) Liprin-α1 does not interfere with the interaction of βPIX with GIT-C2. COS7 cells co-transfected to express the indicated combinations of HA-GIT1-C2, HA-βPIX, and FLAG-liprin-α1 were immunoprecipitated with anti-βPIX antibodies. Filters including aliquots of lysates and the immunoprecipitations (IP) were cut and blotted as indicated. (D) A COS7 cell lysate (1 mg protein) was immunoprecipitated with anti-βPIX antibodies. Immunoprecipitate (IP) and equal amounts (100 µg) of lysate (Lys) and unbound fraction (Ub) were blotted with anti-GIT (mAb PKL, recognizing both GIT1 and GIT2 proteins, on the left; or anti-GIT2-specific pAb, on the right), βPIX, or anti-liprin-α1 antibodies. Blot with anti-GIT antibody was performed after stripping the filter incubated for βPIX. (E) binding of βPIX to full length GIT1 does not enhance the binding of liprin-α1 to GIT1. COS7 cells were co-transfected with FLAG-liprin-α1 and FLAG-GIT1, or with FLAG-liprin-α1 and FLAG-GIT1 and HA-βPIX. 200 µg of each lysate were immunoprecipitated with anti-GIT1 antiserum. Lysates (Lys, 50 µg), unbound fractions (Ub, 50 µg) and immunoprecipitates were blotted and incubated with antibodies specific for the indicated proteins. Overexpression of βPix did not increase the interaction of liprin-α1 with GIT1. (F) Model for the regulated interaction of GIT1 with paxillin and liprin-α1. Either ligand binds poorly to full length GIT1. We hypothesize that activation of GIT1 by so far unknown mechanisms is required for the formation of either GIT1/paxillin or GIT1/liprin-α1 complexes.
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pone-0020757-g001: Binding of liprin-α1 to GIT1-C2 prevents binding of paxillin to GIT1-C2.(A) Lysates were prepared from COS7 cells transfected with either HA-GIT1-C2 (C2) or co-transfected with HA-GIT1-C2 and FLAG-liprin-α1 (C2+Lip). Aliquots of the lysates were used for immunoprecipitation with anti-paxillin antibodies (IP anti-paxillin, 400 µg of protein per IP). Filters with immunoprecipitates (a), and with 100 µg of both lysates (Lys) and unbound fractions after IP (Ub) (b) were cut and immunoblotted with anti-Flag to detect Flag-liprin-α1 (upper filters, only one of the duplicated immunoprecipitations is shown); since GIT1-C2 and paxillin migrate at similar positions on gels, the lower parts of the filters from the duplicated immunoprecipitations were used as follows: one set of filters (a+b) was incubated with anti-HA to detect HA-GIT1-C2 (middle blots), and one set was incubated with anti-paxillin to detect endogenous paxillin (lower blots). Paxillin was absent from the unbound fractions after immunoprecipitation (Ub). (c) The unbound fraction (300 µg) after immunoprecipitation with anti-paxillin from the lysate of cells co-transfected with HA-GIT1-C2 and FLAG-liprin-α1 [Ub(C2+Lip)], was re-immunoprecipitated with anti-liprin antibody, to reveal the presence of the liprin-α1/GIT1-C2 complex in the lysate. (B) Binding of liprin-α1 to GIT1-C2 does not prevent binding of βPIX to GIT1-C2. Identification of a ternary complex among liprin-α1, βPIX and GIT1-C2. COS7 cells co-transfected to express the indicated combinations of HA-GIT1-C2, HA-βPIX, and FLAG-liprin-α1 were immunoprecipitated with anti-FLAG antibodies (top blots on the left). Aliquots of the unbound fraction after the first round of immunoprecipitations were re-immunoprecipitated with anti-βPIX antibodies (top blots on the right). Filters including immunoprecipitations (IP), lysates (Lys), and unbound fractions after the second round of immunoprecipitations (Ub) were cut and blotted as indicated (lower blots). (C) Liprin-α1 does not interfere with the interaction of βPIX with GIT-C2. COS7 cells co-transfected to express the indicated combinations of HA-GIT1-C2, HA-βPIX, and FLAG-liprin-α1 were immunoprecipitated with anti-βPIX antibodies. Filters including aliquots of lysates and the immunoprecipitations (IP) were cut and blotted as indicated. (D) A COS7 cell lysate (1 mg protein) was immunoprecipitated with anti-βPIX antibodies. Immunoprecipitate (IP) and equal amounts (100 µg) of lysate (Lys) and unbound fraction (Ub) were blotted with anti-GIT (mAb PKL, recognizing both GIT1 and GIT2 proteins, on the left; or anti-GIT2-specific pAb, on the right), βPIX, or anti-liprin-α1 antibodies. Blot with anti-GIT antibody was performed after stripping the filter incubated for βPIX. (E) binding of βPIX to full length GIT1 does not enhance the binding of liprin-α1 to GIT1. COS7 cells were co-transfected with FLAG-liprin-α1 and FLAG-GIT1, or with FLAG-liprin-α1 and FLAG-GIT1 and HA-βPIX. 200 µg of each lysate were immunoprecipitated with anti-GIT1 antiserum. Lysates (Lys, 50 µg), unbound fractions (Ub, 50 µg) and immunoprecipitates were blotted and incubated with antibodies specific for the indicated proteins. Overexpression of βPix did not increase the interaction of liprin-α1 with GIT1. (F) Model for the regulated interaction of GIT1 with paxillin and liprin-α1. Either ligand binds poorly to full length GIT1. We hypothesize that activation of GIT1 by so far unknown mechanisms is required for the formation of either GIT1/paxillin or GIT1/liprin-α1 complexes.
Mentions: Based on these findings, we tested the hypothesis that liprin-α1 may interfere with the binding of paxillin to the carboxy-terminus of GIT1 in the cell. For this, we first immunoprecipitated endogenous paxillin from lysates of cells transfected either with HA-GIT1-C2 alone, or with both HA-GIT1-C2 and full length FLAG-liprin-α1. Under conditions in which endogenous paxillin was virtually immunodepleted from lysates (Fig. 1, A, panels a and b), the interaction of paxillin with HA-GIT1-C2 was strongly reduced in the lysates from co-transfected cells (Fig. 1, A, panel a). We then tested the hypothesis that the decrease of binding of paxillin to GIT1-C2 may be due to binding of the overexpressed liprin-α1 to GIT1-C2 itself. For this, the unbound fraction after immunoprecipitation with anti-paxillin from lysates of cells co-transfected with HA-GIT1-C2 and FLAG-liprin-α1, was used in a second round of immunoprecipitation with anti-liprin-α1 antibody (Fig. 1, A, panel c). This immunoprecipitation showed a strong interaction of FLAG-liprin-α1 with HA-GIT1-C2 (Fig. 1, A, panel c). These data suggest that binding of overexpressed liprin-α1 to the carboxy-terminal portion of GIT1 interferes with the binding of paxillin to the same region of GIT1, and indicate that the formation of a trimeric liprin-α1/GIT1/paxillin complex in the cell is not likely.

Bottom Line: Here we have addressed the involvement of the liprin-α1 partner GIT1 in liprin-α1-mediated effects on cell spreading and migration.No cumulative effects were observed after depletion of both liprin-α1 and GIT1, suggesting that the two proteins belong to the same signaling network in the regulation of cell spreading.These findings and the possible competition of liprin-α1 with paxillin for binding to GIT1 suggest that alternative binding of GIT1 to either liprin-α1 or paxillin plays distinct roles in different phases of the protrusive activity in the cell.

View Article: PubMed Central - PubMed

Affiliation: Division of Neuroscience, San Raffaele Scientific Institute and San Raffaele University, Milano, Italy.

ABSTRACT
We have previously identified the scaffold protein liprin-α1 as an important regulator of integrin-mediated cell motility and tumor cell invasion. Liprin-α1 may interact with different proteins, and the functional significance of these interactions in the regulation of cell motility is poorly known. Here we have addressed the involvement of the liprin-α1 partner GIT1 in liprin-α1-mediated effects on cell spreading and migration. GIT1 depletion inhibited spreading by affecting the lamellipodia, and prevented liprin-α1-enhanced spreading. Conversely inhibition of the formation of the liprin-α1-GIT complex by expression of liprin-ΔCC3 could still enhance spreading, although to a lesser extent compared to full length liprin-α1. No cumulative effects were observed after depletion of both liprin-α1 and GIT1, suggesting that the two proteins belong to the same signaling network in the regulation of cell spreading. Our data suggest that liprin-α1 may compete with paxillin for binding to GIT1, while binding of βPIX to GIT1 was unaffected by the presence of liprin-α1. Interestingly, GIT and liprin-α1 reciprocally regulated their subcellular localization, since liprin-α1 overexpression, but not the GIT binding-defective liprin-ΔCC3 mutant, affected the localization of endogenous GIT at peripheral and mature central focal adhesions, while the expression of a truncated, active form of GIT1 enhanced the localization of endogenous liprin-α1 at the edge of spreading cells. Moreover, GIT1 was required for liprin-α1-enhanced haptotatic migration, although the direct interaction between liprin-α1 and GIT1 was not needed. Our findings show that the functional interaction between liprin-α1 and GIT1 cooperate in the regulation of integrin-dependent cell spreading and motility on extracellular matrix. These findings and the possible competition of liprin-α1 with paxillin for binding to GIT1 suggest that alternative binding of GIT1 to either liprin-α1 or paxillin plays distinct roles in different phases of the protrusive activity in the cell.

Show MeSH
Related in: MedlinePlus