Conserved Outer Tegument Component UL11 from Herpes Simplex Virus 1 Is an Intrinsically Disordered, RNA-Binding Protein

Abstract

A distinguishing morphological feature of all herpesviruses is the multiprotein tegument layer located between the nucleocapsid and lipid envelope of the virion. Tegument proteins play multiple roles in viral replication, including viral assembly, but we do not yet understand their individual functions or how the tegument is assembled and organized. UL11, the smallest tegument protein, is important for several distinct processes in replication, including efficient virion morphogenesis and cell-cell spread. However, the mechanistic understanding of its role in these and other processes is limited in part by the scant knowledge of its biochemical and structural properties. Here, we report that UL11 from herpes simplex virus 1 (HSV-1) is an intrinsically disordered, conformationally dynamic protein that undergoes liquid-liquid phase separation (LLPS) in vitro. Intrinsic disorder may underlie the ability of UL11 to exert multiple functions and bind multiple partners. Sequence analysis suggests that not only all UL11 homologs but also all HSV-1 tegument proteins contain intrinsically disordered regions of different lengths. The presence of intrinsic disorder, and potentially, the ability to form LLPS, may thus be a common feature of the tegument proteins. We hypothesize that tegument assembly may involve the formation of a biomolecular condensate, driven by the heterogeneous mixture of intrinsically disordered tegument proteins. IMPORTANCE Herpesvirus virions contain a unique tegument layer sandwiched between the capsid and lipid envelope and composed of multiple copies of about two dozen viral proteins. However, little is known about the structure of the tegument or how it is assembled. Here, we show that a conserved tegument protein UL11 from herpes simplex virus 1, a prototypical alphaherpesvirus, is an intrinsically disordered protein that undergoes liquid-liquid phase separation in vitro. Through sequence analysis, we find intrinsically disordered regions of different lengths in all HSV-1 tegument proteins. We hypothesize that intrinsic disorder is a common characteristic of tegument proteins and propose a new model of tegument as a biomolecular condensate.

Liquid–Liquid Phase Separation by Intrinsically Disordered Protein Regions of Viruses: Roles in Viral Life Cycle and Control of Virus–Host Interactions

“…The tegument protein UL11 from Herpes simplex virus 1 (HSV-1) has been shown to undergo LLPS in vitro, based on macroscopic and time-lapse imaging by a stereo light microscope [196]. The protein is endowed with a high degree of structural disorder and multivalent interaction properties.…”

HSV-1 Cytoplasmic Envelopment and Egress

“…Loss of, or reduced function in, one pathway could then be compensated by enhancing the activity of another. Alternatively, Metrick and colleagues [ 45 ] recently proposed that all inner and outer HSV-1 tegument proteins contain intrinsically disordered regions (IDRs), of variable length, which enable them to undergo phase separation, forming a spherical biomolecular condensate that helps drive envelopment [ 186 ]. If the tegument exists (at least in part) as a biomolecular condensate, it would be extremely tolerant to loss or change in expression level of many of its constituents, and potentially quite variable in its final composition, as multiple tegument or even host proteins could contribute to condensate formation [ 45 , 187 , 188 ].…”

“…Alternatively, Metrick and colleagues [ 45 ] recently proposed that all inner and outer HSV-1 tegument proteins contain intrinsically disordered regions (IDRs), of variable length, which enable them to undergo phase separation, forming a spherical biomolecular condensate that helps drive envelopment [ 186 ]. If the tegument exists (at least in part) as a biomolecular condensate, it would be extremely tolerant to loss or change in expression level of many of its constituents, and potentially quite variable in its final composition, as multiple tegument or even host proteins could contribute to condensate formation [ 45 , 187 , 188 ]. Of possible relevance to this model, we found that the tegument protein UL48p (VP16) ( Table 1 ) binds in vitro to a peptide with the sequence of the gH cytoplasmic tail, but only at 37 °C when that peptide was in a disordered state [ 35 , 36 ].…”

Alphaherpesvirus Major Tegument Protein VP22: Its Precise Function in the Viral Life Cycle

“…The linear dsDNA genome is covered with an icosahedral capsid. The layer of protein between the capsid and outer envelope, collectively termed the tegument, is unique to herpesvirus ( Metrick et al, 2020 ). Twenty-six tegument proteins have been found in HSV-1 ( Loret et al, 2008 ; Kelly et al, 2009 ; Kukhanova et al, 2014 ).…”