mRNA-capping enzyme; HCAP1; HCE; includes: polynucleotide 5'-triphosphatase; mRNA 5'-triphosphatase; TPase; mRNA guanylyltransferase; GTP-RNA guanylyltransferase; GTase (RNGTT, CAP1A)

Function

• bifunctional mRNA-capping enzyme
• RNA 5'-triphosphatase activity in the N-terminal part
• mRNA guanylyltransferase activity in the C-terminal part
• catalyzes the first two steps of mRNA cap formation
  • removing the gamma-phosphate from the 5'-triphosphate end of nascent mRNA to yield a diphosphate end
  • transferring the GMP moiety of GTP to the 5'-diphosphate terminus
• interacts with SUPT5H & RNMT
• interacts with HIV-1 Tat

5'-phosphopolynucleotide + H2O = polynucleotide + phosphate

GTP + (5')pp-pur-mRNA = diphosphate + G(5')ppp-pur-mRNA

Structure

• in the N-terminal section, belongs to the non-receptor class of the protein-tyrosine phosphatase family
• in the C-terminal section; belongs to the eukaryotic GTase family
• isoforms 2,3 & 4 lack mRNA 5'-guanylyltransferase activity due to disruptions of the GTase domain

Compartment

nucleus (putative)

Alternative splicing

named isoforms=4

Expression

• isoform 1 & isoform 4 (at a lesser extent) are expressed in cerebrum, cerebellum, thyroid, lung, heart, liver, kidney, spleen, large intestine, testis, skin & muscle

More general terms

• transferase
• triphosphatase
• phosphoprotein
• nuclear protein

Database

• Entrez gene: http://www.ncbi.nlm.nih.gov/sites/entrez?db=gene&cmd=Retrieve&dopt=Graphics&list_uids=8732
• Kegg: http://www.genome.jp/dbget-bin/www_bget?hsa:8732
• OMIM: https://mirror.omim.org/entry/603512
• UniProt: http://www.uniprot.org/uniprot/O60942.html