Antipeptide antibodies reveal interrelationships of MBP 200 and MBP 235: unique apoB-specific receptors for triglyceride-rich lipoproteins on human monocyte-macrophages

Two human monocyte-macrophage (HMM) membrane binding proteins, (MBP) 200 an d 235, are receptor candidates that bind to the apolipoprotein (apo)B-48 do main in triglyceride-rich lipoproteins for uptake independent of apoE, Micr osequence analysis of the purified reduced MBP 200R characterized tryptic p eptides of MBP 200R. A synthetic peptide mimicking a unique, unambiguous 10 -residue sequence (AEGLMVTGGR) induced antipeptide antibodies that specific ally recognized MBP 200, 235 and 200R, in 1- and 2-dimensional analyses, in dicating 1) the ligand binding protein was sequenced and 2) MBP 200 and 235 yielded MBP 200R upon reduction. These antibodies identified the MBPs in h uman blood-borne, THP-1, U937 MMs, and endothelial cells (EC) but not in hu man fibroblasts or Chinese hamster ovary (CHO) cells. Fluorescence activate d cell sorting (FACS) analysis located the MBPs on the MM surface as necess ary for receptor function, The 10-residue, unambiguous MBP 200-derived sequ ence is unique, with no matches in extant protein databases. Antipeptide an tibodies bind to the MBPs in reticuloendothelial cells that have this recep tor activity, but not to proteins in cells that lack this receptor activity . These studies provide the first direct protein sequence and immunochemica l data that a new, unique apoB receptor for triglyceride-rich lipoproteins exists in human monocytes, macrophages, and endothelial cells.