Icosahedral viruses have been described to infect both
archaea and bacteria, and in both cases the viruses can either have a tail (head-tailed viruses) or be tailless.
Archaeal head-tailed viruses (myo-, sipho-, and podo-
viruses) morphologically resemble such bacteriophages.
Crystallographic analyses have shown that the MCP of
head-tailed phages such as Hong Kong 97 (HK97) forms an
L-shaped molecule consisting of two domains (A and P), an
extended N-terminal arm, and an E-loop (HK97 fold;
Figure 3A).