Insect Sf9 cells are suitable for functional expression of insect, but not vertebrate, striated muscle myosin.

Insect Sf9 cells are widely used for producing recombinant proteins, including myosin. It is expected that the protein folding machinery in Sf9 cells can meet the requirement for the proper folding of exogenous myosin. Of interest is that not all class II myosins are expressed functionally in Sf9 cells. Among vertebrate class II myosins, non-muscle myosin and smooth muscle myosin, but not striated muscle myosin, are functionally expressed in Sf9 cells, presumably due to lacking vertebrate striated muscle myosin-specific chaperone Unc45b in Sf9 cells. Insects only express a generic myosin-specific chaperone Unc45, which is expected to be responsible for the folding of all insect myosins, including striated muscle myosin. This rationale promotes us to investigate the folding of recombinant insect striated muscle myosins in Sf9 cells. We expressed the heavy meromyosin version of the striated muscle myosins from three insect species (Locusta migratoria, Drosophila melanogaster and Plutella xylostella) in Sf9 cells. Similar to vertebrate smooth muscle myosin, but unlike vertebrate striated muscle myosin, the insect striated muscle myosin expressed in Sf9 cells are soluble. The purified recombinant insect striated muscle myosins display normal myosin functions, including ATP-dependent actin interaction, actin-activated ATPase activity, and in vitro actin-gliding activity, indicating that Sf9 cells are suitable for expressing insect striated muscle myosin. We therefore conclude that, unlike vertebrate striated muscle myosin requiring striated muscle-specific chaperones (such as Unc45b) for its folding, insect striated muscle myosin can be properly folded by the generic protein folding machinery in insect cells.