PIM1 is a single gene with 5 introns and 6 exons that span 5 kb of DNA in the human genome (Meeker et al., 1987). The gene starts at 37137922 and ends at 37143204 base pairs from pter. It is highly conserved evolutionarily across species.

The mRNA sequence is 2,6 kb in length with a 941 bp coding region.

Size: 313 amino acids; molecular weight: 36 kDa.
The PIM1 gene has six exons, but there are two isoforms of PIM1 protein, 34 kDa and 44 kDa, due to protein synthesis using alternative sites of translation initiation (Saris et al., 1991). Both proteins show comparable kinase activities in vitro, but the 44 kDa isoform contains an N-terminal proline-rich motif that binds the ETK SH3 domain, and is recruited to the plasma membrane (Xie et al., 2006).

The kinase activity of all the PIM proteins is constitutively active, and there are no regulatory domains in the amino acid sequences of the PIM proteins. Thus, unlike other kinases that are regulated by phosphorylation or binding to the plasma membrane, the activity of PIM1 is regulated primarily by transcription, translation and proteosomal degradation (Amaravadi and Thompson, 2005). The gene expression of PIM1 is increased by various cytokines, mitogens and hormones such as G-CSF, GM-CSF, erythropoietin, interleukins, Con A, PMA, interferons, and prolactin (Wang et al., 2001; Hogan et al., 2008; White, 2003). These factors act through the JAK/STAT pathway. The upregulation of PIM1 gene expression results from the binding site of STAT3 or STAT5 to the PIM1 gene promoter, and the ISFR/GAS-sequence (IFN-γ activation sequence) is an important binding site (Block et al., 2012; Matikainen et al., 1999; Yip-Schneider et al., 1995). PIM1 phosphorylates and stabilizes SOCS proteins (suppressor of cytokine signaling) to provide negative feedback regulation of the JAK/STAT pathway (Peltola et al., 2004).
NF-κB, as a downstream transcription factors, could also activate PIM1. In solid tumors, hypoxia would induce the PIM1 expression, independently of HIF1α and by Krueppel-like factor 5 (KLF5) upon DNA damage (Chen et al., 2009a). ERG, as a transcription factors, also plays a role in PIM1 expression in the initial stages of prostate carcinogenesis (Magistroni et al., 2011).
Because of multiple copies of AUUU(A) motifs in the 3UTR and GC-rich regions in the 5UTR, mRNA of PIM genes are short lived (Wang et al., 2005). Translation of PIM1 seems to be cap-dependent, and overexpression of elF4E would increase PIM1 protein level (Hoover et al., 1997). PIM RNA transcripts are regulatory targets of different miRNAs such as microRNAs miR1, miR-210, miR-33a, and miR328, implicating another layer of PIM expression regulation (Eiring et al., 2010; Huang et al., 2009; Nasser et al., 2008; Thomas et al., 2012).
At the post-translational level, the short half-life of PIM1 is primarily regulated by proteasomal degradation. PIM1 protein can be stabilized by the binding to HSP90 (Mizuno et al., 2001). On the other hand, binding to HSP70 would induce the ubiquitylation of PIM1 and proteasomal degradation (Shay et al., 2005). Also, in hypoxia, ubiquitin-mediated proteasomal degradation of PIM is prevented by HSP90 (Mizuno et al., 2001). PIM1 protein stability is further regulated by its phosphorylation status. PIM1 is able to autophosphorylate (Bullock et al., 2005). Phosphorylation by itself and/or other unknown kinases is important for PIM1 protein stability and function because PP2A phosphatase negatively regulates PIM1 stability (Losman et al., 2003).

PIM1 is highly expressed in lymphoid and hematopoietic tissues (bone marrow, thymus, spleen, and fetal liver) (Eichmann et al., 2000) as well as in some non-hematopoietic tissues (e.g., hippocampus, oral epithelium, and the prostate gland). In some myeloid and lymphoid leukemia cell lines, prostate cancer cell lines, and also HeLa cells, PIM have also been detected. The PIM1 protein can be detected subcellularly in the cytoplasm and the nucleus.

PIM1 phosphorylates a large subset of cellular substrates and thus regulates many different cellular processes such as cell cycle progression, cellular division, differentiation and apoptosis.
One of the cell-cycle-related targets of PIM1 is p21^waf1 (Wang et al., 2002; Zhang et al., 2007). By phosphorylating the CDK inhibitor p21^waf1 on T145, PIM1 lead to the nuclear export and inactivation of p21^waf1. Phosphorylation of another CDK inhibitor p27^Kip1 at Thr157 and Thr198 would induce its proteasomal degradation and cell cycle progression. Moreover, PIM1 seems to phosphorylate and inactivate the transcription factors of p27^Kip1, FoxO1a and FoxO3a (Morishita et al., 2008). Another mechanism of p27^Kip1 regulation is the phosphorylation of SKP2 at Thr417, which control its stability and ability to degrade p27 (Cen et al., 2010). Additionally, the phosphorylation of Cdc25A and Cdc25C would induce G1/S and G2/M transition, respectively (Bachmann et al., 2004). PIM1 also implicate in mitosis promotion by interacts with dynein/dynactin and HP1β (Magnuson et al., 2010).
Moreover, PIM1 is involved in genomic instability. By interaction with NuMA (nuclear mitotic apparatus protein), overexpression of PIM1 causes the loss of checkpoint control (Bhattacharya et al., 2002). Consequently, cells with abnormal mitotic spindles are not arrested in mitosis, producing polyploid and multinucleated daughter cells.
PIM1 can also act as an oncogenic survival factor because of its function in blocking apoptotic cell death. It is consensus that phosphorylation of BAD at S112 would induce its proteasomal degradation and thus shifts the apoptosis threshold (Peltola et al., 2004). The proapoptotic activity of ASK1 and PRAS40 would also be impaired by PIM1 phosphorylation (Gu et al., 2009; Zhang et al., 2009). Through inactivation of ASK1 and subsequently less phosphorylation of the stress kinases JNK and p38, caspase-3 activation would be less and thus reduce cell death. Moreover, the block of MDM2 and p53 degradation by PIM1 may induce senescence in embryonic fibroblasts and cancer cells (Hogan et al., 2008).
When bound to MYC at the E-box, PIM1 would participate in the complexs phosphorylation of histone H3 at S10 and thus participate in the stimulation of transcription of a specific subset of MYC-dependent genes (Zippo et al., 2007).
Additionally, PIM1 influences the activity of a number of transcriptional regulators, such as HP-1, PAP-1, TFAF2/SNX6, NFATc1, p100, RUNX, SOCS1, RelA/p65 and c-Myb (Bhattacharya et al., 2002; Evans and Fox, 2007; Ishibashi et al., 2001; Kim et al., 2010; Rainio et al., 2002; Winn et al., 2003).
PIM kinases also phosphorylate 4E-BP1, inhibiting its binding to elF-4E. Since elF-4E is a rate-limiting factor in protein synthesis, PIM kinases may also regulate 5 cap-dependent translation (Beharry et al., 2011).