Probable principal replicase for the negative-strand DNA, which replicates the 40S (+) genomic RNA into (-) antigenomic RNA. It cannot replicate the (-) into (+) until cleaved into p150 and p90 mature proteins. Protease that cleaves the precursor polyprotein into two mature products. Together with RNA-directed RNA polymerase p90, replicates the 40S genomic and antigenomic RNA by recognizing replications specific signals. The heterodimer P150/p90 is probably the principal replicase for positive-strand genomic RNA and the 24S subgenomic RNA, which codes for structural proteins. Responsible for the mRNA-capping of the viral mRNAs. This function is necessary since all viral RNAs are synthesized in the cytoplasm, and host capping enzymes are restricted to the nucleus. Forms fibers late in the infection that may be involved in cell-to-cell spread of the virus RNA in the absence of virus particle formation. Together with protease/methyltransferase p150, replicates the 40S genomic and antigenomic RNA by recognizing replications specific signals. The heterodimer P150/p90 is probably the principal replicase for positive-strand genomic RNA and the 24S subgenomic RNA, which codes for structural proteins. A helicase activity is probably also present. a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) a ribonucleoside 5'-triphosphate + H2O = a ribonucleoside 5'-diphosphate + H(+) + phosphate ATP + H2O = ADP + H(+) + phosphate Zn(2+) is necessary for the protease activity. The protease can also function efficiently with Cd(2+) and Co(2+). Interacts with RNA-directed RNA polymerase p90. Interacts with host CALM1; this interaction is necessary for the protease activity and viral infectivity. Interacts with host C1QBP. Interacts with the capsid protein. Interacts with human RB1/retinoblastoma protein. Interacts with protease/methyltransferase p150. Localizes to cytoplasmic foci at 24 hpi. At 36 hpi, localizes to the host cytoplasm, probably in vesicles inside host vacuoles of endosomal and lysosomal origin (By similarity). At 72 hpi, localizes to filamentous structures in the host cytoplasm (By similarity). Localizes to the cytoplasm and to the cytoplasmic fibers formed by protease/methyltransferase p150. The N-terminus has a methyltransferase activity for mRNA-capping. The C-terminus harbors a protease active in cis or in trans which specifically cleaves and releases the two mature proteins. Both the N-terminus and C-terminus are required for fiber formation. The N-terminus is involved in associating with membranes. An EF-hand Ca(2+)-binding motif is present in the protease. Also contains 3 SH3-binding motifs that are responsible for the interaction with host C1QBP. Specific enzymatic cleavage by its own cysteine protease yield mature proteins p150 and p90. Rubella virus in utero infection has frequently severe consequences on normal fetal development, collectively known as congenital rubella syndrome (CRS). The teratogenicity of the virus is possibly due to the interaction between the p90 protein and the human RB1/retinoblastoma protein.