Determination of the tyrosine phosphorylation sites in the T cell transmembrane glycoprotein CD5

Area of Science:

• Immunology
• Molecular Biology
• Cell Signaling

Background:

• The transmembrane glycoprotein CD5 plays a critical role in negatively regulating antigen receptor-mediated signals in various immune cells, including T cells and B1a cells.
• CD5 possesses four tyrosine residues in its cytoplasmic domain, which undergo phosphorylation upon antigen receptor ligation, a process implicated in its signaling function.
• It has been proposed that CD5's function is mediated by the recruitment of the tyrosine phosphatase SHP-1 to phosphorylated CD5, leading to dephosphorylation of signaling molecules.

Purpose of the Study:

• To investigate the specific requirements for CD5 tyrosine phosphorylation.
• To identify the precise sites of CD5 tyrosine phosphorylation.
• To elucidate the role of the tyrosine kinase p56(lck) in CD5 phosphorylation.

Main Methods:

• Utilized a T cell line deficient in p56(lck) and a reconstituted version to assess the kinase's requirement for CD5 phosphorylation.
• Employed tyrosine-phosphorylated peptides of CD5 cytoplasmic sequences to analyze the binding affinity of the p56(lck) Src homology 2 (SH2) domain.
• Generated and expressed murine CD5 tyrosine-to-phenylalanine (Y --> F) mutants and deletion mutants in Jurkat T cells to map phosphorylation sites.

Main Results:

• p56(lck) expression was found to be essential for efficient CD5 tyrosine phosphorylation.
• The SH2 domain of p56(lck) exhibited strong binding to the pY429SQP peptide, with weaker affinity for pY463DLQ and no binding to pY441PAL.
• Mutational analysis indicated that Y429 and Y463 are the primary sites for CD5 tyrosine phosphorylation, as Y429F and Y463F mutants showed reduced phosphorylation, while Y441F did not.
• Deletion mutants lacking Y378 suggested this residue is not readily available for phosphorylation.

Conclusions:

• Both Y429 and Y463 residues are critical for recruiting p56(lck) and represent the main sites of CD5 tyrosine phosphorylation.
• p56(lck) is required for efficient CD5 tyrosine phosphorylation, highlighting its role in regulating CD5 signaling.
• These findings provide a detailed understanding of the molecular mechanisms governing CD5 phosphorylation and its implications in immune cell regulation.