LCKBP1, A PROLINE-RICH PROTEIN EXPRESSED IN HEMATOPOIETIC LINEAGE CELLS, DIRECTLY ASSOCIATES WITH THE SH3 DOMAIN OF PROTEIN-TYROSINE KINASEP56(LCK)

The Lck tyrosine kinase molecule plays an essential role in T cell act ivation and T cell development. Using the expression cloning technique , we have isolated a gene that encodes a molecule, LckBP1, able to ass ociate with murine Lck. Analysis of full-length LckBP1 cDNA indicates at least four potentially important segments: a four tandem 37 amino a cid repeat motif with a potential helix-turn-helix DNA binding motif; a proline-rich region; a proline-glutamate repeat; and an SH3 domain, These four regions are very similar to the human haematopoietic-specif ic protein 1 (HS1), Deletion mutant analysis of LckBP1 revealed two pr oline-rich regions that permit association with Lck SH3. One region co ntains prolines conserved among HS1 and cortactin, and the other regio n contains a potential MAP kinase recognition site. In vivo associatio n between Lck and LckBP1 was confirmed by immunoprecipitation of lysat es from a pre-T cell line and adult thymocytes using antibodies specif ic for Lck and LckBP1. LckBP1 is tyrosine phosphorylated after T-cell receptor stimulation, The SH3 domain and the potential helix-turn-heli x motif in LckBP1 suggest that this molecule may associate with variou s molecules and function as a DNA binding molecule. The data also sugg est that LckBP1 mediates intracellular signalling through Lck in T cel ls.