AN ATYPICAL SORTING DETERMINANT IN THE CYTOPLASMIC DOMAIN OF P-SELECTIN MEDIATES ENDOSOMAL SORTING

We previously identified the 11 amino acid C1 region of the cytoplasmi c domain of P-selectin as essential for an endosomal sorting event tha t confers rapid turnover on P-selectin. The amino acid sequence of thi s region has no obvious similarity to other known sorting motifs. We h ave analyzed the sequence requirements for endosomal sorting by measur ing the effects of site-specific mutations on the turnover of P-select in and of the chimeric protein LLP, containing the lumenal and transme mbrane domains of the low density lipoprotein receptor and the cytopla smic domain of P-selectin. Endosomal sorting activity was remarkably t olerant of alanine substitutions within the C1 region. The activity wa s eliminated by alanine substitution of only one amino acid residue, l eucine 768, where substitution with several other large side chains, h ydrophobic and polar, maintained the sorting activity. The results ind icate that the endosomal sorting determinant is not structurally relat ed to previously reported sorting determinants. Rather, the results su ggest that the structure of the sorting determinant is dependent on th e tertiary structure of the cytoplasmic domain.