Prosomes form sarcomere-like banding patterns in skeletal, cardiac, and smooth muscle cells

Prosomes (20S proteasomes) constitute the catalytic core of the 26S proteas omes, but were first observed as factors associated with unstranslated mRNA . Recently, their RNase activity was discovered together with the fact that their proteolytic function is dispensable in adapted human cells. By indir ect immunofluorescence using monoclonal antibodies, we demonstrate as a gen eral phenomenon, regular intercalation of specific types of prosomes into t he sarcomeric structure of all types of striated muscle. Surprisingly, in c ultured smooth muscle cells without sarcomeric organization, some prosomes also form regular striations in extended projections of cytoplasmic regions . The significance of their sarcomeric distribution is not understood as ye t, but the pattern we observe is very similar to that shown by others for m uscle-specific mRNAs, identified by in situ hybridization, and that of the cognate proteins. A role of prosomes in the cotranslational assembly of the myofibrillar proteins is suggested, since prosomes organize into pseudo-sa rcomeric patterns prior to formation de novo of the actin-myosin arrangemen t. (C) 2001 Academic Press.