hTERC is transcribed as a precursor and is processed at its 3' end to yield a 451 nucleotide RNA (Zaug et al. 1996). The accumulation of hTERC that has undergone this processing event requires a conserved region of sequence termed the box H/ACA motif (Mitchell et al. 1999a). This motif is bound by a complex containing dyskerin, and mutations in dyskerin affect the processing and accumulation of hTERC (Mitchell et al. 1999b; Mitchell and Collins 2000; Fu and Collins 2003). Recent studies of purified, catalytically active telomerase indicate that the minimal structure that has telomerase activity in vitro is a complex of one molecule of hTERC RNA and two each of hTERT and DKC1 (dyskerin) proteins (Cohen et al. 2007). Several additional proteins may associate with this minimal complex and modulate its activity. RUVBL1 (pontin), RUVBL2 (reptin), and TCAB1 (telomere Cajal body protein 1) are found associated with human telomerase RNPs purified from HeLa cells, and activities of these proteins are required for telomerase RNP assembly in vivo (Venteicher et al. 2008, 2009). NHP2 (NOLA2) is likewise associated with telomerase ribonucleoprotein complexes (Pogacic et al. 2000) and homozygosity for NHP2 mutations is associated with telomerase failure (dyskeratosis congenita) in humans (Vuillamy et al. 2008). The exact roles of the additional proteins in the assembly and function of telomerase RNP in vivo remain unclear, however, so they are annotated simply as positively regulating telomerase RNP formation.
The core components hTERC and hTERT undergo trafficking in the cell that may be important for telomerase function. hTERC has been found localized in multiple nuclear structures, including Cajal bodies, nucleoli, and at telomeres (Mitchell et al. 1999a; Jady et al. 2004; Zhu et al. 2004; Jady et al. 2006; Tomlinson et al. 2006). hTERT is also reported localize in Cajal bodies, nucleoli, and to associate with telomeres (Etheridge et al. 2002; Wong et al. 2002; Yang et al. 2002; Zhu et al. 2004; Tomlinson et al. 2006). Some of the factors that regulate trafficking of these two core components of telomerase have been identified, such as nucleolin (Khurts et al. 2004), SMN (Bachand et al. 2002), and 14-3-3 (Seimiya et al. 2000). Cytological studies of HeLa cells suggest that the localization of the telomerase core components can change through the cell-cycle (Jady et al. 2006; Tomlinson et al. 2006). Despite these studies, it is not clear in which compartment hTERT and hTERC assemble to form functional telomerase RNP.
The assembly of telomerase involves the chaperone proteins p23 and Hsp90, which stably associate with telomerase in vitro (Holt et al. 1999; Forsythe et al. 2001; Keppler et al. 2006). A number of other proteins interact with the telomerase RNP, but it is not clear if they play a role in telomerase assembly. Interestingly, assembled human telomerase RNP can multimerize, though the function of multimerization remains unclear (Beattie et al. 2001; Wenz et al. 2001; Arai et al. 2002).