Mechanism of length determination in bacteriophage lambda tails.

The mechanism of length determination in bacteriophage lambda tails is discussed as a model for regulation in protein assembly systems. The lambda tail is a long flexible tube ending in a conical part and a single tail fiber. Its length is exactly determined in the sense that the number of major tail protein (gpV) molecules, which comprise more than 80% of the mass of the tail, is exactly the same in all tails. Assembly of gpV is regulated by the initiator complex, which contains the tail fiber and the conical part, and by the terminator protein gpU. There are two key points in the assembly of gpV with respect to length determination. (1) Assembly of gpV on the initiator pauses at the correct tail length. Binding of gpU to the tail only fixes the pause firmly. (2) When the tail length is too short, binding of gpU to tails is inhibited. Deletions and a duplication (both in frame) in gene H, which codes for one of the proteins in the initiator, result in production of phage particles with altered tail length. Moreover, the tail length is roughly proportional to the length of the mutated versions of gene H. This shows that the tail length is measured by the length of gene H protein (gpH), which seems to be approximately as long as the tail tube, if extended like a thread, according to secondary structure prediction (alpha-helices connected by other structures). Various pieces of evidence show that about six molecules of gpH are attached to the remaining portion of the initiator by the C-terminal part and folded into a somewhat compact form, while they are elongated as they are enclosed in the tail tube during assembly of gpV. Unlike interaction between the length-measuring genome RNA and the coat protein of tobacco mosaic virus, the major tail protein gpV does not bind specifically to the ruler protein gpH. Rather, gpH determines the tail length by inhibiting the binding of gpU to short tails and by signalling the pause when the correct tail length is attained.