The human HRK gene spans 20206 bases, telomere to centromere orientation. Exon 2, which encodes the 3 UTR of the HRK mRNA, is much longer in the mouse and rat genes compared to human HRK (see Figure 2A). In the three species only one major transcript encoding a single protein isoform has been described.

In northern blotting experiments with RNA from human tissues, it was reported that the HRK transcript was detected in spleen, lymph nodes, thymus, bone marrow and appendix (Inohara et al., 1997). However, in the rat and mouse the expression of the HRK transcript is much more restricted and HRK is detected in the brain but not the spleen, thymus, bone marrow, liver, lung, testis, heart, intestine or skeletal muscle (Imaizumi et al., 1997; Coultas et al., 2007).
Based on studies with the rat HRK gene (Ma et al., 2007; Towers et al., 2009), the 1 kb region upstream of exon 1 contains elements important for the control of HRK transcription (Figure 2B). The HRK promoter is GC-rich but contains a conserved block of 14 A/T nucleotides that might function as a TATA box, a conserved E-box, a conserved GC box and a conserved and functionally-important ATF binding site (5-TGATGTAA-3) that binds c-Jun and ATF2 and which is important for the activation of HRK transcription by the JNK pathway following survival factor withdrawal in neurons (Ma et al., 2007; Towers et al., 2009) or exposure to pro-inflammatory cytokines in pancreatic beta-cells (Gurzov et al., 2009).

There are no known pseudogenes for HRK.

Only one isoform of the HRK protein has been described. HRK is 91 amino acids long in humans and 92 amino acids in mouse and rat (Inohara et al., 1997; Imaizumi et al., 1997; Imaizumi et al., 1999).

See section on transcription for information about tissue specificity. The endogenous HRK protein has been detected in immunoblotting experiments with a number of cell types, for example: in NGF-deprived rat primary sympathetic neurons in culture (Imaizumi et al., 1997), in the mouse brain following focal cerebral ischemia (middle cerebral artery occlusion; Gao et al., 2005), in the auditory cell line HEI-OC1 exposed to gentimicin (Kalinec et al., 2005) and in the pancreatic beta-cell line INS-1E treated with the pro-inflammatory cytokines IL-1beta and IFNgamma (Gurzov et al., 2009).

HRK is a non-nuclear intracellular protein (Inohara et al., 1997). Flag-tagged HRK co-localizes with MitoTracker in transfected COS-7 cells suggesting that HRK predominantly localizes to mitochondria (Sunayama et al., 2004). Studies with a 27 amino acid peptide containing the putative transmembrane domain of HRK indicated that this domain is able to insert into membranes, where it adopts a transmembrane alpha-helical structure (Bernabeu et al., 2007). This suggests that the carboxy terminal region of HRK may insert into the mitochondrial outer membrane.

HRK is a pro-apoptotic BH3-only member of the Bcl-2 protein family. Overexpression of HRK can induce apoptosis in HEK293 cells (Inohara et al., 1997; Imaizumi et al., 1999), rat sympathetic neurons (Imaizumi et al., 1997) and cerebellar granule neurons (Harris and Johnson, 2001). In these cell types HRK/DP5-induced apoptosis is blocked by co-expression of Bcl-2 or Bcl-xL (Inohara et al., 1997; Imaizumi et al., 1997) or by knockout of the endogenous Bax gene (Harris and Johnson, 2001). The HRK protein contains two functional domains: a BH3 domain and a carboxy terminal transmembrane domain (Figure 3). The HRK BH3 domain is related in amino acid sequence to the BH3 domains of other Bcl-2 family proteins and is required for interaction with anti-apoptotic Bcl-2 family proteins, such as Bcl-2 and Bcl-xL, and for cell death induced by overexpression of HRK (Inohara et al., 1997). A detailed study of the binding of BH3-only proteins to anti-apoptotic Bcl-2 family members indicated that HRK binds with high affinity to Bcl-xL, Bcl-w and A1 and with moderate affinity to Bcl-2 and Mcl-1 (Chen et al., 2005). The HRK transmembrane domain is rich in hydrophobic amino acid residues and could mediate the insertion of the HRK carboxy terminus into intracellular membranes, such as the mitrochondrial outer membrane.

The HRK protein is only related to other Bcl-2 family proteins in the short BH3 domain.

No mutations have been described in HRK.