Folding of coliphage T4 short tail fiber in vitro

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Abstract
The morphogenesis of the Escherichia coli bacteriophage T4 depends on the presence of helper proteins which are not components of the mature virion. Two bacteriophage‐encoded proteins, p57 and p38, are required for the assembly of the bacteriophage T4 tail fibers. In the absence of p57, two polypeptides of the long fiber (p34 and p37) and that of the short tail fiber (p12) fail to trimerize. Instead they form water‐insoluble aggregates. Co‐expression of the genes 12 and 57 in vivo caused the formation of only trimeric, water‐soluble p12. The function of g57 cannot be replaced by overexpression of the host proteins GroEL/ES or parvulin. The mechanism of action of this helper protein has remained unknown, mainly because it has not been possible to determine its activity in vitro. Purified p12, denatured in 7 m urea, trimerized spontaneously in a slow reaction (half‐time ≈ 6 h) and with low yield. Upon renaturation, p12 forms native SDS‐resistant trimers as indicated by spectroscopic and hydrodynamic measurements. Addition of p57 increased the rate of folding threefold and nearly doubled the yield. These experiments demonstrate that p57 acts as a molecular chaperone during folding of T4 tail fibers.