HSV capsid/organelle association in the absence of the large tegument protein UL36p

UL36p (VP1/2) is the largest protein encoded by herpes simplex virus 1 (HSV-1) and resides in the innermost layer of the viral tegument, lying between the capsid and the envelope. UL36p performs multiple functions in the HSV life cycle, including an essential role in cytoplasmic envelopment. We earlier described the isolation of a virion-associated cytoplasmic membrane fraction from HSV-infected cells. Biochemical and ultrastructural analyses showed that the organelles in this buoyant fraction contain enveloped infectious HSV particles in their lumens and naked capsids docked to their cytoplasmic surfaces. These organelles can also recruit molecular motors and transport their cargo virions along microtubules in vitro. Here we examine the properties of these HSV-associated organelles in the absence of UL36p. We find that while capsid envelopment is clearly defective, a sub-population of capsids nevertheless still associate with the cytoplasmic faces of these organelles. The existence of these capsid-membrane structures was confirmed by subcellular fractionation, immunocytochemistry, lipophilic dye fluorescence microscopy, thin-section electron microscopy, and correlative light and electron microscopy. We conclude that capsid-membrane binding can occur in the absence of UL36p and propose that this association may precede the events of UL36p-driven envelopment. IMPORTANCE Membrane association and envelopment of the HSV capsid are essential for the assembly of an infectious virion. Envelopment involves the complex interplay of a large number of viral and cellular proteins; however, the function of most of them is unknown. One example of this is the viral protein UL36p, which is clearly essential for envelopment but plays a poorly understood role. Here we demonstrate that organelles utilized for HSV capsid envelopment still accumulate surface-bound capsids in the absence of UL36p. We propose that UL36p-independent binding of capsids to organelles occurs prior to the function of UL36p in capsid envelopment.