EVIDENCE THAT APO-B-100 OF LOW-DENSITY LIPOPROTEINS IS A NOVEL SRC-RELATED PROTEIN-KINASE

Protein-tyrosine kinases of signal transduction pathways occur and fun ction intracellularly. In contrast, the low-density lipoprotein (LDL) particle circulates in plasma, where its function is to solubilize and transport lipid. Recently, several reports showed that LDL may have a role in signal transduction. We have identified a region in the apoB- 100 primary structure which shows similarity to Src-homology-1 (SH1) d omains, the kinase region of protein-tyrosine kinases. Results obtaine d in protein kinase assays of highly purified LDL showed that only the apoB-100 was phosphorylated, suggesting that apoB-100 has the capacit y to undergo autophosphorylation like known protein-tyrosine kinases. Phosphorylation was not observed for any other apolipoprotein in LDL o r for any component of high-density lipoprotein and lipoprotein [a]. O ur results suggest that apoB-100 may be a novel and functional member of the src protein kinase family.