HBV persistence and transmission require HBV replication, which depends on the assembly of a core particle composed of capsid protein (Cp), polymerase, and pregenomic RNA. Reverse transcription to produce infectious DNA-containing particles occurs solely within the core residing in the cytoplasm (9, 10). Thus, core assembly is likely to be a high value target for therapeutics (11).The capsid, the protein shell of the core, is built of 120 Cp dimers arranged with T ϭ 4 symmetry (12, 13). The dimer interfaces are evident as spikes (14-16) that are the major epitope of the capsid (17). Cp in low ionic strength solution is dimeric (18). We have studied ionic strength-dependent capsid assembly extensively in vitro by using the Cp assembly domain (Cp149) (residues 1-149) lacking the 34-residue C-terminal RNA-binding domain (19)(20)(21). Assembly is nucleated by a trimer of Cp dimers and proceeds without accumulating observable populations of intermediates (22). Interactions between dimers are weak but sum to give a globally stable capsid (23). These capsids persist, even under conditions where they are not thermodynamically favored, because of hysteresis to dissociation (24). Some Cp mutations lead to faster assembly and greater stability (25), indicating that wild-type Cp is suboptimal for assembly and suggesting that assembly is regulated in vivo, possibly by conformational change. In support of this assertion, we found that Zn 2ϩ alters the conformation of Cp dimers and enhances the rate of assembly, suggesting that capsid assembly is allosterically regulated (26).Recently, it was discovered that heteroaryldihydropyrimidines (HAPs) (Fig. 1) affect the accumulation of HBV capsids (27,28). HAP drugs decreased the yield of assembled core and HBV genomes from cells that constitutively produce HBV. Electron microscopy showed that Cp assembled in vitro in the presence of HAP drugs led to polymers that had abnormal morphology (29). Similarly, small molecules such as bis ANS {5,5-bis[8-(phenylamino)-1-naphthalenesulfonate]} alter Cp assembly in vitro (30). Recent reports suggest that other small molecules also inhibit normal HBV capsid assembly (31-33).Here, we describe the mechanism of a representative HAP compound, HAP-1 [methyl 4-(2-chloro-4-f luorophenyl)-6-methyl-2-(pyridin-2-yl)-1,4-dihydropyrimidine-5-carboxylate] (Fig. 1). In vitro, low concentrations of HAP-1 enhance both the rate and extent of assembly by favoring an assembly-active state; thus, HAP-1 acts like an allosteric effector. At higher concentrations, HAP-1 led to aberrant noncapsid polymers in vitro, even at the expense of preexisting capsids. We propose that both of these effects on assembly contribute to reducing HBV virion production. Materials and MethodsSynthesis of HAP-1. Preparation of racemic HAP-1 (Fig. 1) was adapted from the patent literature (27,28,34). Condensation of a pyridylamidine with a substituted ␣-carboxymethyl enone gave a 30% yield of Ͼ97% pure HAP-1 after chromatographic purification. HAP-1 was characterized by 1 H, 13 C, and 19 ...

A critical feature of a viral life cycle is the ability to selectively package the viral genome. In vivo, phosphorylated hepatitis B virus (HBV) core protein specifically encapsidates a complex of pregenomic RNA (pgRNA) and viral polymerase; it has been suggested that packaging is specific for the complex. Here, we test the hypothesis that core protein has intrinsic specificity for pgRNA, independent of the polymerase. For these studies, we also evaluated the effect of core protein phosphorylation on assembly and RNA binding, using phosphorylated core protein and a phosphorylation mimic in which S155, S162, and S170 were mutated to glutamic acid. We have developed an in vitro system where capsids are disassembled and assembly-active core protein dimer is purified. With this protein, we have reassembled empty capsids and RNA-filled capsids. We found that core protein dimer bound and encapsidated both the HBV pregenomic RNA and heterologous RNA with high levels of cooperativity, irrespective of phosphorylation. In direct competition assays, no specificity for pregenomic RNA was observed. This suggests that another factor, such as the viral polymerase, is required for specific packaging. These results also beg the question of what prevents HBV core protein from assembling on nonviral RNA, preserving the protein for virus production.

Here we report the effect of a heteroaryldihydropyrimidine (HAP) antiviral compound, BAY 41-4109, on Hepatitis B virus (HBV) capsid assembly and on preformed HBV capsids. The HBV capsid is an icosahedral complex of 120 capsid protein dimers. BAY41-4109 inhibits virus production in vivo by a mechanism that targets the viral capsid. We found that BAY 41-4109 was able to both accelerate and misdirect capsid assembly in vitro. As little as one HAP molecule for every five HBV dimers was sufficient to induce formation of non-capsid polymers. Unlike the related molecule HAP-1 (Stray et al., Proc. Natl. Acad. Sci. USA 102:8138-43, 2005), no stable assembly intermediates were observed in assembly reactions with BAY 41-4109, indicating that accelerated assembly by BAY 41-4109 was still kinetically regulated by the nucleation rate. Preformed capsids were stabilized by BAY 41-4109, up to a ratio of one inhibitor molecule per two dimers. However, at BAY 41-4109:dimer ratios of 1:1 and greater, capsids were destabilized to yield very large non-capsid polymers. These data suggest the existence of two functionally distinguishable classes of drug-binding sites on HBV capsids. Occupation of the first class of site stabilizes capsid, while binding at the second class requires or induces structural changes that cannot be tolerated without destabilizing the capsid. Our data suggest that HAP compounds may inhibit virus replication by inducing assembly inappropriately and, when in excess, by misdirecting assembly decreasing the stability of normal capsids.