T4 helper protein p57 (gp57A) is a highly alpha-helical (~80% alpha-helix) nonstructural chaperone required for the assembly of p12 (short tail fiber), p34 (proximal long tail fiber), and—together with p38—p37 (distal long tail fiber).
"In the absence of the other, p57, p34 (another component of the long tail fiber), p37, and p12 (the subunit of the short tail fiber) remain unassembled."