This is the N-terminal domain found in the symplekin protein from animals and PTA1 protein from budding yeasts and is typically between 239 to 261 amino acids in length. Symplekin is a scaffold protein that functions as a component of a multimolecular complex involved in histone mRNA 3'-end processing [1,2]. PTA1 is involved in pre-tRNA processing. PTA1 is a subunit of the cleavage and polyadenylation factor (CPF), which plays a key role in polyadenylation-dependent pre-mRNA 3'-end formation and cooperates with cleavage factors including the CFIA complex and NAB4/CFIB [3]. This domain has the ARM or HEAT fold, with seven pairs of antiparallel alpha-helices arranged in the shape of an arc. It is important for interaction with Ssu72 and stimulates Ssu72 C-terminal domain phosphatase activity in vitro [4].

1: A genome-wide RNA interference screen reveals that variant histones are necessary for replication-dependent histone pre-mRNA processing. Wagner EJ, Burch BD, Godfrey AC, Salzler HR, Duronio RJ, Marzluff WF; Mol Cell. 2007;28:692-699. PMID:18042462

2: A core complex of CPSF73, CPSF100, and Symplekin may form two different cleavage factors for processing of poly(A) and histone mRNAs. Sullivan KD, Steiniger M, Marzluff WF; Mol Cell. 2009;34:322-332. PMID:19450530

3: Organization and function of APT, a subcomplex of the yeast cleavage and polyadenylation factor involved in the formation of mRNA and small nucleolar RNA 3'-ends. Nedea E, He X, Kim M, Pootoolal J, Zhong G, Canadien V, Hughes T, Buratowski S, Moore CL, Greenblatt J; J Biol Chem. 2003;278:33000-33010. PMID:12819204

4: Crystal structure of the human symplekin-Ssu72-CTD phosphopeptide complex. Xiang K, Nagaike T, Xiang S, Kilic T, Beh MM, Manley JL, Tong L; Nature. 2010;467:729-733. PMID:20861839