Bacteriophage Mu head assembly.

The protein composition of defective particles produced by various bacteriophage Mu head-gene mutants was analyzed by SDS-PAGE. An abundant 20-kDa protein was detected in only one type of defective head. This protein exhibits properties of a scaffolding protein. A 50-kDa structural protein present in most defective heads was shown to be produced by cleavage of the C-terminus of the 64-kDa polypeptide encoded by gene H. Cleavage occurs during head assembly at a site which, according to earlier results, might separate two different functional domains in gpH. A fraction of the gpH molecules produced upon Mu induction sediment in a 25 S complex, suggesting that gpH participates in the formation of an early intermediate of Mu head assembly. Characteristics of gpH suggest that it may be the Mu portal protein.