Since it had been observed (5) that phosphorylation was
necessary for the unpurified cellular protein to bind the RV
RNA, we analyzed the ability of Cal-MBP to bind RV RNA
after phosphorylation in vitro. Both purified Cal-MBP and
MBP were incubated with [y-32P]ATP in the presence or
absence of Vero 76 cytosolic extracts and labeled proteins
were immunoprecipitated by MBP antibody. The MBP antibody
did not precipitate any labeled protein from cell
extracts alone (Fig. 4C, lane 5). The Cal-MBP was 32p
labeled (Fig. 4C, lanes 2 and 4), whereas purified MBP failed
to be labeled under similar conditions