INVOLVEMENT OF PROTEASOMAL SUBUNIT-ZETA AND SUBUNIT-IOTA IN RNA DEGRADATION

We have identified two distinct subunits of 20 S proteasomes that are associated with RNase activity. Proteasome subunits zeta and iota, elu ted from two-dimensional Western blots, hydrolysed tobacco mosaic viru s RNA, whereas none of the other subunits degraded this substrate unde r the same conditions. Additionally, proteasomes were dissociated by 6 M urea, and subunit zeta, containing the highest RNase activity, was isolated by anion-exchange chromatography and gel filtration. Purified subunit zeta migrated as a single spot on two-dimensional PAGE with a molecular mass of approx. 28 kDa. Addition of anti-(subunit zeta) ant ibodies led to the co-precipitation of this proteasome subunit and nuc lease activity. This is the first evidence that proteasomal alpha-type subunits are associated with an enzymic activity, and our results pro vide further evidence that proteasomes may be involved in cellular RNA metabolism.