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 DKC1 (dyskerin), and mutations in dyskerin affect the processing and accumulation of hTERC (Mitchell et al. 1999b; Mitchell and Collins 2000; Fu and Collins 2003). 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, one molecule of hTERT and two molecules of DKC1 (dyskerin) (Cohen et al. 2007). A cryo-electron microscopy (EM) structure of human substrate-bound telomerase holoenzyme revealed that, in addition to one molecule of hTERC RNA, one molecule of hTERT and two molecules of DKC1, the holoenzyme also contains one molecule of WRAP53 (TCAB1, also known as telomere Cajal body protein 1) and two molecules of each NOP10, NHP2 (NOLA2) and GAR1 (Nguyen et al. 2018). WRAP53 is needed for the activity and localization of the telomerase holoenzyme to Cajal bodies (Venteicher et al. 2009). Homozygosity for NHP2 mutations is associated with telomerase failure (dyskeratosis congenita) in humans (Vuillamy et al. 2008). Several additional proteins may associate with the holoenzyme, promoting its assembly and modulating its activity. RUVBL1 (pontin) and RUVBL2 (reptin) 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). Pontin and reptin may modulate the interaction between SHQ1 and DKC1 (Machado-Pinilla et al. 2012), but as their exact roles in the assembly and function of telomerase RNP remain unclear, they are annotated simply as positive regulators of 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).