A novel human apolipoprotein (apoM)

A novel human apolipoprotein designated apolipoprotein IM (apoM) is describ ed. The unique N-terminal amino acid sequence of apoM was found in an appro ximately 26-kDa protein present in a protein extract of triglyceride-rich l ipoproteins (TGRLP). The isolated apoM cDNA (734 base pairs) encoded a 188- amino acid residue-long protein, distantly related to the lipocalin family. The mRNA of apoM was detected in the liver and kidney. Western blotting de monstrated apoM to be present in high density lipoprotein (HDL) and to a le sser extent in TGRLP and low density lipoproteins (LDL). The first 20 amino acid residues of apoM constituted a hydrophobic segment with characteristi c features of a signal peptide. This was retained in the mature protein bec ause of the lack of a signal peptidase cleavage site, In vitro translation in the presence of microsomes demonstrated translocation of apoM over the m embrane and glycosylation but no signal peptide cleavage. The in vitro tran slated product remained associated with the microsomes after treatment with carbonate at pH 11, demonstrating that apoM is an integral protein. This f inding suggests that apoM is linked to the single phospholipid layer of lip oproteins with a hydrophobic signal anchor. In conclusion, a novel human ap olipoprotein, the function of which remains to be determined, is described.