The 25 amino acid residues at the carboxy terminus of the herpes simplex virus type 1 UL26.5 protein are required for the formation of the capsid shell around the scaffold

Abstract

Herpes simplex virus type 1 (HSV-1) polypeptides specified by overlapping genes UL26 and UL26.5 form a scaffold around which the icosahedral capsid shell is assembled. In a series of cleavage events catalysed by the UL26-encoded protease, the full-length UL26 product is processed into capsid proteins VP24 and VP21 and the UL26.5 protein is converted into the capsid protein VP22a by the loss of 25 amino acids from its carboxy terminus. The roles of the UL26 and UL26.5 products were investigated using the baculovirus expression system, focusing on the function of the 25 residues cleaved from the UL26.5 protein. A key conclusion from electron microscopic analysis and protein expression studies is that the 25 amino acids at the carboxy terminus of the full-length UL26.5 protein are required for the interaction of the capsid shell proteins with the scaffold in the formation of intermediate capsids. When cells were multiply infected with baculoviruses expressing a truncated form of the UL26.5 product corresponding to VP22a and the essential components of the capsid shell, no capsids were detected, whereas large numbers of capsids were observed when the full-length UL26.5 product was used as a scaffold. The results are consistent with the proposal that cleavage of the UL26.5 product occurs after capsid assembly or when the UL26.5 protein is in a complex with one or more capsid shell proteins. Expression of VP22a in the absence or presence of capsid shell proteins resulted in the formation of large numbers of 60 nm scaffold-like particles. Since VP22a expressed from baculovirus was unable to participate in capsid assembly, these particles cannot be intermediates in the capsid assembly pathway but may be similar in structure to the protein cores present in HSV-1 immature (B) capsids.

Multiple Interactions Control the Intracellular Localization of the Herpes Simplex Virus Type 1 Capsid Proteins

“…This seems unlikely since the fluorescence data showed no interaction between VP23 and preVP22a (Nicholson et aL, 1994). We have also observed 70 nm particles in cells expressing VPS, VP19C, preVP22a and the UL26 protease, but not in cells expressing VPS, VP19C and preVP22a, where indistinct spherical structures similar to those formed by VP5 and preVP22a alone (Kennard et al, 1995) were found (unpublished data). This suggests that removal by the UL26 protease of the C-terminal 25 amino acids of preVP22a, which are known to be necessary for interaction with VP5…”

“…The lack of a direct interaction between VP5 and VP23 is more surprising but is supported by our earlier studies (Nicholson et al, 1994) and by the absence of observable structures in cells coinfected with baculoviruses expressing VP5 and VP23. The formation, by VP5 and VP19C alone, of uniform 70 nm particles which are clearly related to the outer shell of normal capsids, also suggests that VP23 is not essential for the interaction between triplex and capsomer and in addition Nicholson et of., 1994) and electron microscope analysis (this paper; Tatman et al, 1994;Thomsen et ol., 1994;Kennard et aL, 1995) are shown by the solid lines. The gene encoding each protein is shown in brackets below the protein name.…”

“…'.25~ (Kennard et al, 1995 ;Matusick-Kumar et al, 1995 ;Hong et al, 1996), prevents this interaction thereby allowing VP5 and VP19C to form the 70 nm particles.…”

Impaired Assembly of E1 Decarboxylase of the Branched-chain α-Ketoacid Dehydrogenase Complex in Type IA Maple Syrup Urine Disease

“…The important roles of the C terminus in subunit assembly and protein interactions have been demonstrated. For example, the C-terminal 25 amino acid residues of the herpes simplex virus type 1 UL26.5 protein are required for the assembly of the icosahedral capsid shell (26). In the case of the E2 core of the related pyruvate dehydrogenase complex from Azotobacter vinelandii, the C-terminal residues 632-637 comprise a 3 10 -like helix (H6) which acts as a "hydrophobic knob" that fits into a "hole" in the 2-fold related subunit to produce the 24-mer cubic assembly (27).…”

“…Aromatic residues from E1␤ form a hydrophobic pocket to accommodate the pyrimidium and thiazolium rings of cofactor TPP. On the other hand, the highly conserved TPP-binding motif GDG(X) [22][23][24][25][26][27][28] NN, which was first described by Hawkins et al (35) and is essential for binding the pyrophosphate moiety, is located in the E1␣ subunit (Fig. 8).…”