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Expression of Cowpea Mosaic Virus M RNA in Cowpea Protoplasts

Department of Molecular Biology, Agricultural University, Dreijenlaan 3, 6703 HA Wageningen, The Netherlands

Cowpea mosaic virus (CPMV) M RNA is translated in vitro into two polyproteins of M_r values 105000 (105K) and 95K. Using antiserum against the small capsid protein VP23, these proteins have now been detected in cowpea protoplasts, a few hours after inoculation with CPMV. These proteins could also be detected at later stages of infection, but only when proteolytic processing was inhibited by the addition of ZnCl₂. Using antiserum against a synthetic peptide, corresponding to a part of the overlapping C-terminal ends of the 58K and 48K proteins, the 58K protein, which is the amino-terminal cleavage product of the 105K protein, was found in the cytoplasmic fraction of infected protoplasts. The 48K protein, derived from the 95K protein, was detected in both the cytoplasmic and membrane fractions of protoplasts. The presence of the 105K, 95K, 58K and 48K proteins in CPMV-infected protoplasts indicates that separate initiation codons on the M RNA are used in vivo to produce the 105K and 95K polyproteins, as demonstrated in vitro.

Keywords: CPMV, M RNA, protoplasts

Received 14 March 1989; accepted 7 July 1989.