The T-antigen proteins of the three previously known polyomaviruses are highly conserved and T-antigens from SV40 and BKV are >70% identical in amino acid (aa) content (Box 1). BKV and JCV T-antigens possess an indentity of 83%. Despite the high degree of conservation between BKV, JCV and SV40, clearly the non-identical aas are crucial. T-antigens of SV40 and JCV will support replication of polyomavirus DNA in murine cells while BKV T-antigen will not [19
]. The three newly discovered viruses are different. The large T-antigens of JCV and WUV are only 49% identical to each other. Furthermore, the MCV large T-antigen is distinct among those of the human polyomaviruses. All of the other T-antigens are only about 30% identical to that of MCV, and this is largely due to conservation among a few critical elements.
LPV T-antigen domains are not strongly homologous to those of any of the human polyomaviruses, including MCV. However, overall the LPV T-antigen is slightly more homologous to those of KIV, WUV and JCV than is that of MCV although the identities are <45% in all cases.
The first 100 aa of all the T-antigens are well conserved and include the J domain. This domain is similar to the DnaJ chaperone protein of Escherichia coli
and interacts with the ATPase domain of the mammalian HSP70 chaperone [20
]. The J domain includes the sequence HPDKGG, which is precisely conserved in T-antigens of all five human polyomaviruses and SV40. The T-antigen Rb-binding domain contains a Psycho domain that modulates Rb activities [23
] and the box LXCXE, which is important for binding of T-antigen to a pocket region of the Rb protein [25
]. The Psycho domain of the MCV T-antigen is interrupted by 106 aas containing 40 S or T residues (). This S-rich “spacer” separates the LFCDE motif from the rest of the domain, in contrast to all of the other T-antigens. The effect of this separation on Rb binding and Rb functional inactivation by MCV T-antigen remains to be determined. SV40 and MCV have been linked to naturally-occurring neoplasias in primates. It is thus of interest that T-antigens of these viruses differ markedly in their Rb-binding configurations. LPV T-antigen is not included in . It is considerably shorter than the other T-antigens, and it lacks a Psycho domain. It does have the HPDKGG J-element and an LFCSE Rb-binding element.
The p53- and DNA-binding sites of T-antigens comprise multiple overlapping elements. Major p53-binding elements are located in aa position 123 to 215, corresponding to SV40 numbering, and DNA binding elements (not shown) are located in positions 123 to 246, depending on various reports. This region is moderately conserved, with specific motifs identical in all of the T-antigens. Moreover, the p53 binding regions of BKV and JCV are 90% identical in sequence (Box 1). BKV, JCV and SV40 p53/DNA-binding regions are 54-56% identical to that of MCV. KIV and WUV are most different from MCV in their p53 binding regions, at 42% and 43%, respectively.
All of the T-antigens, including that of LPV, possess highly-conserved Walker A and B boxes [26
] in their helicase domains. The BKV, JCV and SV40 helicase domains are all at least 90% identical in sequence (Box 1). T-antigen helicase domains from all the human polyomaviruses range from 60% to 84% in identity. Furthermore, the Walker A box, an ATP-binding motif, is present as KGPI(N/D)SGKT in every example. KIV and WUV are 78% identical to each other in the helicase region, and are thus almost as closely related in this region as are BKV and JCV (84%).
It is clear that the polyomaviral large T-antigens differ widely amongst each other, with MCV most distinct. It will be valuable to relate disease capacity to the ability of T-antigens to bind p53 and DNA.