The CERV protein of Cer1, a C. elegans LTR retrotransposon, is required for nuclear export of viral genomic RNA and can form giant nuclear rods

Abstract

Retroviruses and closely related LTR retrotransposons export full-length, unspliced genomic RNA (gRNA) for packaging into virions and to serve as the mRNA encoding GAG and POL polyproteins. Because gRNA often includes splice acceptor and donor sequences used to splice viral mRNAs, retroelements must overcome host mechanisms that retain intron-containing RNAs in the nucleus. Here we examine gRNA expression in Cer1, an LTR retrotransposon in C. elegans which somehow avoids silencing and is highly expressed in germ cells. Newly exported Cer1 gRNA associates rapidly with the Cer1 GAG protein, which has structural similarity with retroviral GAG proteins. gRNA export requires CERV ( C . e legans regulator of viral expression), a novel protein encoded by a spliced Cer1 mRNA. CERV phosphorylation at S214 is essential for gRNA export, and phosphorylated CERV colocalizes with nuclear gRNA at presumptive sites of transcription. By electron microscopy, tagged CERV proteins surround clusters of distinct, linear fibrils that likely represent gRNA molecules. Single fibrils, or groups of aligned fibrils, also localize near nuclear pores. During the C. elegans self-fertile period, when hermaphrodites fertilize oocytes with their own sperm, CERV concentrates in two nuclear foci that are coincident with gRNA. However, as hermaphrodites cease self-fertilization, and can only produce cross-progeny, CERV undergoes a remarkable transition to form giant nuclear rods or cylinders that can be up to 5 microns in length. We propose a novel mechanism of rod formation, in which stage-specific changes in the nucleolus induce CERV to localize to the nucleolar periphery in flattened streaks of protein and gRNA; these streaks then roll up into cylinders. The rods are a widespread feature of Cer1 in wild strains of C. elegans, but their function is not known and might be limited to cross-progeny. We speculate that the adaptive strategy Cer1 uses for the identical self-progeny of a host hermaphrodite might differ for heterozygous cross-progeny sired by males. For example, mating introduces male chromosomes which can have different, or no, Cer1 elements.

Author summary

LTR retrotransposons are closely related to retroviruses and are enormously abundant in animals and plants. Cer1 is the most prevalent LTR retrotransposon in the nematode C. elegans, where it is expressed at high levels in adult germ cells. Cer1 produces typical retroviral proteins except for a novel protein called CERV. CERV appears to allow unspliced Cer1 genomic RNA to be exported from the nucleus, escaping host mechanisms that normally retain unspliced RNA. The nuclear export of gRNA is regulated by multiple ON/OFF switches controlled by sex, developmental stage, and environment. CERV is a diffuse nucleoplasmic protein in the OFF states, but in the ON states CERV colocalizes with nuclear gRNA. By transmission electron microscopy, CERV is associated with unusual and distinct linear fibrils that appear to represent gRNA molecules. In older germ cells, CERV undergoes a remarkable transition to form giant cylindrical rods of unknown function that can equal the nuclear diameter in length. Rod formation occurs when C. elegans hermaphrodites can no longer fertilize their own oocytes, and instead require mating with males. Thus, the rods might be part of adaptive strategies Cer1 uses to distinguish potentially heterozygous cross-progeny from homozygous self-progeny.