DNA topoisomerase I
The molecular weight of enzymatically-active topoisomerase I determined by SDS-electrophoresis has been reported to range from 67-100 kDa. Recent data suggest that the 100 kDa form is the predominant form in vivo, and that the lower molecular weight forms are due to proteolytic degradation. Both the native and lower molecular weight forms have been shown to effectively bind anti-Scl-70 autoantibodies.
The amino acid sequence of human Scl-70 antigen (DNA topoisomerase I) has recently been described after cloning and sequencing its cDNA. The deduced molecular weight of the mature protein is 90,649 kDa; lower than the SDS-electrophoretic molecular weight. This may be due to post-translational processing or to anomalous electrophoretic behaviour of the protein. The protein was found to be very basic, with a calculated isoelectric point of 10.05, and to possess its catalytic activity within a 67.7 kDa C-terminal fragment.
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