THE TYROSINE-BASED LYSOSOMAL TARGETING SIGNAL IN LAMP-1 MEDIATES SORTING INTO GOLGI-DERIVED CLATHRIN-COATED VESICLES

Diversion of membrane proteins from the trans-Golgi network (TGN) or t he plasma membrane into the endosomal system occurs via clathrin-coate d vesicles (CCVs). These sorting events may require the interaction of cytosolic domain signals with clathrin adaptor proteins (APs) at the TGN (AP-1) or the plasma membrane (AP-2). While tyrosine- and di-leuci ne-based signals in several proteins mediate endocytosis via cell surf ace CCVs, segregation into Golgi-derived CCVs has so far only been doc umented for the mannose 6-phosphate receptors, where it is thought to require a casein kinase II phosphorylation site adjacent to a di-leuci ne motif, Although recently tyrosine-based signals have also been show n to interact with the mu chain of AP-1 in vitro, it is not clear if t hese signals also bind intact AP-1 adaptors, nor if they can mediate s orting of proteins into AP-1 CCVs. Here we show that the cytosolic dom ain of the lysosomal membrane glycoprotein lamp-1 binds AP-1 and AP-2. Furthermore, lamp-1 is present in AP-1-positive vesicles and tubules in the trans-region on the Golgi complex, AP-1 binding as ell as local ization to AP-1 CCVs require the presence of the functional tyrosine-b ased lysosomal targeting signal of lamp-1. These results indicate that lamp-1 can exit the TGN in CCVs and that tyrosine signals can mediate these sorting events.