The QM Protein Associates with Ribosomes in the Rough Endoplasmic Reticulum

Abstract

QM is a human cDNA originally isolated as a transcript elevated in a nontumorigenic Wilms' tumor microcell hybrid, relative to the tumorigenic parental cell line. Homologs of this gene have been identified from a large number of diverse eukaryotic species which demonstrate a high degree of conservation. The functional importance implied by this strong conservation is supported by the observation that the disruption of the yeast homolog is lethal. In spite of its apparent importance, the function of the encoded protein remains elusive. Indirect immunofluorescent cell staining of cultured human, G401 cells with an antibody to the QM protein shows a punctate staining pattern in the cytoplasm with much of the signal in a perinuclear pattern. Subcellular fractionation demonstrated an association of QM protein with the rough endoplasmic reticulum. It was possible to disrupt this association by washing microsomal membranes with 1M NaCl, suggesting a peripheral association. Proteolytic latency studies showed the protein to be exposed on the cytoplasmic face of the membrane. In situ cross-linking followed by diagonal SDS gel analysis indicates that QM exists as a member of a large protein complex. In agreement with this, QM was found to copurify with the ribosome complex. Incubation with 1 M NaCl was found to disrupt this association while having no effect on the association of core ribosomal proteins.