The highly conserved HP1 family proteins play an important role in the dynamic organization of nuclear architecture and in the epigenetic control of gene expression. Although most species contain more than one HP1 family member which differ in their chromosomal distribution, it is not known to what extent their function is redundant or specific in a developmental context.
C. elegans has two HP1 homologues, HPL-1 and HPL-2. While
hpl-2 is required for the maintenance of a functional germline and for vulval development by acting in the Rb related synMuvB pathway, no obvious function has so far been attributed to HPL-1. We report the characterization of an
hpl-1 null allele. We show that while the absence of
hpl-1 alone results in no obvious phenotype,
hpl-1;
hpl-2 double mutants show synthetic, temperature sensitive phenotypes including larval lethality and severe defects in the development of the somatic gonad. Furthermore, we find that
hpl-1 has an unexpected role in vulval development by acting redundantly with
hpl-2, but not other genes previously implicated in vulval development. By using reporter transgenes and quantitative real-time RT-PCR analysis, we have identified several direct or indirect targets of
hpl-2. Localization studies show that like HPL-2, HPL-1 is a ubiquitously expressed nuclear protein. However, HPL-1 and HPL-2 show clear differences in their localization. This is most clearly evident in embryos, where HPL-1 and HPL-2 are found concentrated in non-overlapping nuclear foci. We find that the LIN-13 protein, which forms a complex with HPL-2, is absolutely required for recruitment of HPL-2, but not HPL-1, in these foci. Our results show that HPL-1 and HPL-2 play both unique and redundant functions in post-embryonic development. Altogether, these studies contribute to an understanding of the function of HP1 proteins in gene regulation throughout development, and in a more general way on how a general chromatin interacting protein may play specific roles in given developmental pathways