In previous studies it was shown that a DNA-free bacteriophage T7 procapsid (capsid I) draws DNA to its interior during assembly and as it begins this packaging capsid I changes in structure to capsid II. To probe the structures of capsids I and II, they have been digested with trypsin. The most abundant protein in the envelopes of capsids I and II, P10, was cleaved when capsid I was digested, but was not cleaved when capsid II was digested. Cleavage of capsid I-associated P10 formed a major P10 fragment 18% reduced in molecular weight, but did not detectably affect the size of capsid I (determined from sieving during agarose gel electrophoresis) or appearance of capsid I in the electron microscope. The magnitude of the (negative) average electrical surface charge density of capsid I was increased by digestion with trypsin (determined by agarose gel electrophoresis). At least one minor P10 fragment remained associated with capsid I after digestion.
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