Kinase Family PIM
Individual Notes
Pim1 has an unusual start to the protein, in mouse and probably other species. A careful study by Saris et al [1] showed that two proteins are made by the same mouse cDNA. a 34kD band is associated with a traditional ATG start codon (protein start = MLLSKINSLA), while a 44 kD initiates upstream in the same exon, at a CTG (Leucine) codon, extending the protein by 84 AA (start = MGPAAPLALP). Mutation or deletion of the CTG abolished the longer isoform.
The human gene also has a long 5' UTR in the same exon as the ORF start, has a CTG codon at the same position as in mouse, and the upstream extension of the gene is moderately similar to that of mouse. The Saris paper cites unpublished data that only one human protein isoform was seen in the the tissue they tested, it's possible that a longer human form is also expressed..
The long UTR of PIM1 (including the putative upstream protein extension) is highly conserved in DNA, forms a predicted RNA secondary structure (free energy of -153 kCal/mol) and appears to control translation of the protein [2]. Deletion of the region in human (including the predicted upstream extension above the ATG) resulted in ~10-fold increase in in vitro translation (and a modest in vivo effect), and this effect was modulated by changing eIF-4E levels. A subsequent paper [3]identified human Pim1 in a screen for transcripts with cap-independent translation and showed that the 5' UTR contains an internal ribosome entry sites (IRES). This apparent function of the UTR complicates our ability to predict whether an extension of the ORF into this region is conserved for protein-coding regions or otherwise. Conversely, the IRES may assist with the non-canonical translation start.
Strong conservation in the UTR, and genomic region is seen beyond this extended start, indicating that the extended ORF is not the only functional element in this region.
Analysis of other vertebrates is largely supportive, though several draft assemblies (horse, cow, marmoset, guinea pig) have gaps just upstream of the ATG, possibly due to difficulty in cloning and maintenance of sequences with high secondary structure.
The shorter PIM1 protein (ATG start) aligns well to the start of the PIM3 protein in human and mouse.
References
- Saris CJ, Domen J, and Berns A. The pim-1 oncogene encodes two related protein-serine/threonine kinases by alternative initiation at AUG and CUG. EMBO J. 1991 Mar;10(3):655-64. DOI:10.1002/j.1460-2075.1991.tb07994.x |
- Hoover DS, Wingett DG, Zhang J, Reeves R, and Magnuson NS. Pim-1 protein expression is regulated by its 5'-untranslated region and translation initiation factor elF-4E. Cell Growth Differ. 1997 Dec;8(12):1371-80.
- Johannes G, Carter MS, Eisen MB, Brown PO, and Sarnow P. Identification of eukaryotic mRNAs that are translated at reduced cap binding complex eIF4F concentrations using a cDNA microarray. Proc Natl Acad Sci U S A. 1999 Nov 9;96(23):13118-23. DOI:10.1073/pnas.96.23.13118 |
