Anomalies in the Translocation and Processing of Glycophorin Precursors in Murine Erythroleukemia Cells

Analogues of human glycophorins have been identified in many species, including mouse. In murine erythroleukemia cells three mature glycophorins (gp-2, gp-3, 34K) and four putative precursors (21K, 23K, 26K, and 27K) are expressed. Pulse-chase labeling experiments suggest that 21K and 23K are the precursors of the gp-3 doublet of proteins 29K and 30K. Precursor-product relationships were not found for 26K, 27K, 34K, and gp-2. Two experimental approaches, i.e. cell-free translation and subcellular distribution, have identified processing anomalies in the glycophorins. First, signal sequences, if present, are apparently not cleaved upon translocation across the endoplasmic reticulum membrane; second, the 26K and 27K putative precursors are inefficiently translocated. The majority accumulates in the cytosol or associates with the endoplasmic reticulum membrane without acquiring protection against V8 protease. Only a minority (approximately 10%) is properly translocated.