Heterogeneous nuclear ribonucleoprotein K (alsoprotein K) is aprotein that in humans is encoded by theHNRNPKgene.[5] It is found in thecell nucleus that binds to pre-messenger RNA (mRNA) as a component ofheterogeneous ribonucleoprotein particles. Thesimianhomolog is known asprotein H16. Both proteins bind to single-strandedDNA as well as toRNA and can stimulate the activity ofRNA polymerase II, the protein responsible for mostgene transcription. The relative affinities of the proteins for DNA and RNA vary with solution conditions and are inversely correlated, so that conditions promoting strong DNA binding result in weak RNA binding.[6]
RNA binding protein domains in other proteins that are similar to the RNA binding domain of protein K are calledK-homology orKH domains.
This gene belongs to the subfamily of ubiquitously expressedheterogeneous nuclear ribonucleoproteins (hnRNPs). The hnRNPs areRNA-binding proteins, and they complex withheterogeneous nuclear RNA (hnRNA). These proteins are associated withpre-mRNAs in the nucleus and appear to influence pre-mRNA processing and other aspects ofmRNA metabolism and transport. While all of the hnRNPs are present in the nucleus, some seem to shuttle between the nucleus and the cytoplasm.
The hnRNP proteins have distinct nucleic acid binding properties. The protein encoded by this gene is located in thenucleoplasm and has three repeats ofKH domains that binds to RNAs. It is distinct among other hnRNP proteins in its binding preference; it binds tenaciously to poly(C). This protein is also thought to have a role during cell cycle progression. Multiple alternatively spliced transcript variants have been described for this gene, but only three variants have been fully described.[8]
Mutations in both copies ofHNRNPK are embryonic lethal in mice. Mice with both copies of the gene knocked out die before the 14th day ofembryonic development.[9]
Deletions in the region encompassingHNRNPK have been found in the cells ofacute myeloid leukemia in approximately 2% of cases. Additionally, a majority of mice who have had one of theirHNRNPK genes artificiallyknocked out developedmyeloid cancers, with a third developinglymphoid cancers and 4% developinghepatocellular carcinomas. The mice were also smaller, had less developed organs and had higher postnatal mortality (30%). The median lifespan of the mice that survived was less than 50% that of wild-type mice. Deficiencies in HNRNPK appear to specifically reduce the levels of the p42isoform ofCEBPA, which is atranscription factor involved in the differentiation of certainblood cells, as well asp21 (cyclin-dependent kinase inhibitor 1), which is involved in pausing cell development forDNA repair.[11]
HNRNPK overexpression also appears to contribute to cancers via a different mechanism involvingtranslation rather thantranscription.[11]
^"Human PubMed Reference:".National Center for Biotechnology Information, U.S. National Library of Medicine.
^"Mouse PubMed Reference:".National Center for Biotechnology Information, U.S. National Library of Medicine.
^Dejgaard K, Leffers H, Rasmussen HH, Madsen P, Kruse TA, Gesser B, et al. (February 1994). "Identification, molecular cloning, expression and chromosome mapping of a family of transformation upregulated hnRNP-K proteins derived by alternative splicing".Journal of Molecular Biology.236 (1):33–48.doi:10.1006/jmbi.1994.1116.PMID8107114.
^abcKim JH, Hahm B, Kim YK, Choi M, Jang SK (May 2000). "Protein-protein interaction among hnRNPs shuttling between nucleus and cytoplasm".Journal of Molecular Biology.298 (3):395–405.doi:10.1006/jmbi.2000.3687.PMID10772858.
^Yang JP, Reddy TR, Truong KT, Suhasini M, Wong-Staal F (October 2002). "Functional interaction of Sam68 and heterogeneous nuclear ribonucleoprotein K".Oncogene.21 (47):7187–94.doi:10.1038/sj.onc.1205759.PMID12370808.S2CID26091319.
^Wada K, Inoue K, Hagiwara M (August 2002). "Identification of methylated proteins by protein arginine N-methyltransferase 1, PRMT1, with a new expression cloning strategy".Biochimica et Biophysica Acta (BBA) - Molecular Cell Research.1591 (1–3):1–10.doi:10.1016/s0167-4889(02)00202-1.PMID12183049.
Buckanovich RJ, Posner JB, Darnell RB (October 1993). "Nova, the paraneoplastic Ri antigen, is homologous to an RNA-binding protein and is specifically expressed in the developing motor system".Neuron.11 (4):657–72.doi:10.1016/0896-6273(93)90077-5.PMID8398153.S2CID22554933.
Tommerup N, Leffers H (March 1996). "Assignment of human KH-box-containing genes by in situ hybridization: HNRNPK maps to 9q21.32-q21.33, PCBP1 to 2p12-p13, and PCBP2 to 12q13.12-q13.13, distal to FRA12A".Genomics.32 (2):297–8.doi:10.1006/geno.1996.0121.PMID8833161.
Kim JH, Hahm B, Kim YK, Choi M, Jang SK (May 2000). "Protein-protein interaction among hnRNPs shuttling between nucleus and cytoplasm".Journal of Molecular Biology.298 (3):395–405.doi:10.1006/jmbi.2000.3687.PMID10772858.
Overview of all the structural information available in thePDB forUniProt:P61978 (Human Heterogeneous nuclear ribonucleoprotein K (HNRPK)) at thePDBe-KB.