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. 2005 May 15;105(10):3918-24.
doi: 10.1182/blood-2004-09-3689. Epub 2005 Feb 8.

Rapid ubiquitination of Syk following GPVI activation in platelets

Carol A Dangelmaier  1 , Patricia G Quinter, Jianguo Jin, Alexander Y Tsygankov, Satya P Kunapuli, James L Daniel
Affiliations

Rapid ubiquitination of Syk following GPVI activation in platelets

Carol A Dangelmaier et al. Blood. .

Abstract

Spleen tyrosine kinase (Syk) activation is a key intermediate step in the activation of platelets by the physiologic agonist collagen. We have found that Syk is rapidly ubiquitinated upon activation of platelets by collagen, collagen-related peptide (CRP), and convulxin. The Src family kinase inhibitors prevented Syk phosphorylation and its ubiquitination, indicating that the process is downstream of Src kinases. The ubiquitination of Syk did not cause degradation of the protein as evidenced by the lack of effect of proteasomal and lysosomal inhibitors. We separated ubiquitinated Syk from its nonubiquitinated counterpart and used an in vitro kinase assay to compare their activities. We found that the ubiquitinated Syk appeared to be about 5-fold more active. Using a phosphospecific antibody to Syk (Tyr525/Tyr526) that measures activated Syk, we found that most (60%-75%) of the active Syk is in the ubiquitinated fraction. This result explains the apparent high specific activity of ubiquitinated Syk. In c-Cbl-deficient mice, Syk is not ubiquitinated, implicating c-Cbl as the E3 ligase involved in Syk ubiquitination. Furthermore, Syk is not dephosphorylated in these mice. We propose that c-Cbl plays a regulatory role in glycoprotein VI (GPVI)/Fc receptor gamma (FcRgamma)-chain-dependent platelet activation through its interaction with Syk.

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Figures

Figure 1.
Figure 1.
Phosphotyrosine-containing proteins in c-Cbl immunoprecipitates of platelets stimulated with convulxin. Platelets were stimulated with 100 ng/mL convulxin for the indicated times and lysed with NP-40 lysis buffer. Proteins were immunoprecipitated with an antibody to c-Cbl and probed with an antibody to phosphotyrosine. Blots were analyzed on a Fuji LAS-1000 plus imaging system. This experiment and those shown in all other figures are representative of at least 3 similar experiments. Stds indicates protein molecular weight standards.
Figure 2.
Figure 2.
The ubiquitination of Syk in activated human platelets. (A) Syk was immunoprecipitated from RIPA-lysed platelets that had been activated with convulxin for the indicated times. The immunoprecipitates were separated by SDS-PAGE and transferred to Immobilon. The blot was first probed for ubiquitin (antibody P4D1; Ai). This blot was stripped and probed for Syk (Aii). The same samples were run on another gel and transferred to Immobilon and probed for phosphotyrosine (Aiii). (B) Syk was immunoprecipitated from lysates of activated platelets with thrombin (T, lanes 2-4), thrombin plus SC57101 (T+, lanes 6-8), or convulxin (column 9) and probed for phosphotyrosine. (C) Platelets were preincubated for 10 minutes with 20 μM PP2 as indicated prior to activation with convulxin. Syk was immunoprecipitated and the blots were probed either for Syk (Ci) or for phosphotyrosine (Cii).
Figure 3.
Figure 3.
Neither proteasome inhibitors nor lysosomal inhibitors affect the levels of ubiquitinated Syk. (A) Washed platelets were pretreated with the proteasomal inhibitor epoxomicin (1 μM 30 minutes at 37°C) prior to activation by convulxin (100 ng/mL) for indicated times. The platelets were lysed with RIPA buffer and Syk was immunoprecipitated. The blot was probed for Syk. (B) Epoxomicin and MG262 inhibit platelet proteasomal enzymes. Washed platelets were incubated with either epoxomicin (1 μM) or MG262 (1 μM) for 30 minutes at 37°C. The platelets were washed and lysed as described. The lysates were assayed for proteasomal activity with Suc-Leu-Leu-Val-Tyr-AMC as substrate. (C) Measurement of the effect of epoxomicin (○) on ubiquitinated Syk using a phosphospecific (pY525/pY526) antibody. The experiment was similar to panel A, except the Syk immunoblot was probed with phosphospecific (pY525/pY526) antibody. The total material in the Syk and ubiquitinated Syk bands was quantitated for each incubation time and the area plotted in the figure. ▪ indicates control. The inset shows the 2 blots used. The data are representative of 3 experiments. (D) Washed platelets were pretreated with chloroquine (200 μM for 60 minutes at 37°C), a lysosomal inhibitor, prior to activation by collagen for indicated times. The platelets were lysed and Syk was immunoprecipitated. The blots were probed for Syk.
Figure 4.
Figure 4.
Separation of ubiquitinated Syk using an antiubiquitin antibody. Washed platelets were activated with convulxin (100 ng/mL) for 15 seconds. Cells were lysed with RIPA buffer and treated with FK2 antibody for 1 hour at 4°C, followed by Protein A/G PLUS-agarose for an additional hour. Supernatants were treated with anti-Syk using the same protocol. Aliquots were analyzed by Western blotting probing for Syk. Lane A shows total Syk; lane B, the second immunoprecipitate after the supernatant was treated with anti-Syk; and lane C, the material immunoprecipitated by FK2.
Figure 5.
Figure 5.
Phosphorylation of tubulin by FK2 immunoprecipitates. (A) Washed platelets were activated with convulxin and lysed in NP-40 buffer. Lysates were treated with FK2 antibody for 1 hour at 4°C followed by 1 hour with Protein A/G PLUS-agarose. The immunoprecipitates were assessed by an in vitro kinase assay in the presence of DMSO (• and —) or 2 μM PP2 (▵ and —) or 200 μM piceatannol (□ and –). The inset shows the blot from which these data were derived. (B) The FK2 fraction of convulxin-activated platelets (• and —) and the total Syk fraction (▵ and –) were assayed for their kinase activity as in Figure 4. The relative protein mass in each fraction was assessed from the immunoblots and activity calculated based on the relative mass. Since the mass of the total Syk was about 10-fold higher than the polyubiquitinated fraction, we also assayed the total Syk fraction after 10-fold dilution (□ and —).
Figure 6.
Figure 6.
Active Syk is enriched in the ubiquitinated fraction. (A) An immunoblot of ubiquitinated Syk probed with an antibody to total Syk (lane 1) is compared with a blot of ubiquitinated Syk probed with a phosphospecific (pY525/pY526) antibody (lane 2). (B) The FK2 fraction of convulxin-activated platelets (○ and —) and the Syk fraction obtained after the majority of the ubiquitinated form was removed (▪ and —) were assayed for their kinase activity as in Figure 5. The relative protein mass in each fraction was assessed from immunoblots using the phosphospecific antibody for Syk residues pY525/pY526. Error bars indicate standard error of three determinations.
Figure 7.
Figure 7.
Aggregation and Syk activation in wild type (+/+) and c-Cbl knock-out (-/-) murine platelets. Washed murine platelets were activated for indicated times with 100 ng/mL convulxin (Cvx). (A) Aggregation tracings are shown (the arrow indicates addition of Cvx). (B) Syk was immunoprecipitated as described and immunoblots were probed for phosphotyrosine. This antibody was stripped from the blot and reprobed for Syk (not shown). IP indicates immunoprecipitation.
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