FIG. 4.
Altered thermodynamics of ASLV-A envelope triggering. (A) Increased protease sensitivity of SU subunit reveals receptor-triggered conformational change in envelope. Soluble receptor was bound to virus on ice where indicated, then the temperature was either shifted to 37°C or maintained on ice for 15 min as shown. Samples were returned to ice, digested with thermolysin, and resolved by SDS-PAGE and anti-SU Western blot. The positions of unprocessed (SU) and cleaved (SU*) subunit are shown. (B) Some of the mutant envelopes are efficiently triggered by receptor without elevating temperature. The bands from three separate gels were quantitated using [125I]protein A and the ratio of the amount of SU* detected at 2°C compared to 37°C was calculated. Shown are the average ratio and standard deviation for each envelope. The dashed line at a ratio of 1.0 indicates temperature independence.