Baculovirus
infection of Sf9 cells at high densities, such as during mid- and late exponential phase, often results in a significant reduction of
protein yield per cell, compared to the early exponential phase. Nutrient depletion has been considered as a major cause for the decreased
protein yield. In this study, we report that the addition of nutrients (
glucose, yeastolate ultrafiltrate, and
lactalbumin hydrolysate) and small fraction of fresh medium at time of
infection restores the expression level of actin and
myosin V-HMM at late exponential phase (11.3 × 10(6) cells/ml) to that at early exponential phase (1.0 × 10(6) cells/ml). The relative yields of actin and
myosin V-HMM were approximately equal at both phases (typically 200 mg of actin and 5 mg of
myosin V-HMM per 10(10) cells), i.e., the volumetric yield of
proteins from the cell culture at late exponential phase was approximately tenfold higher than at early exponential phase. The functionality of the recombinant actin and
myosin V-HMM was confirmed by measuring the rate of actin polymerization,
actin-activated ATPase, and the gliding velocity of actin filaments in an in vitro motility assay.