LIG_ANK_PxLPxL_1

Ankyrin repeat domains (PF00023) are a common protein module in eukaryotic proteomes. They are named after the archetypal protein ankyrin, a key component of the membrane-associated cystoskeleton.They are comprised of short, tandem repeating motifs, with variable repeat numbers ranging from 2 to 34. Each basic repeating core unit consists of 33 amino residues and adopts a canonical helix-turn-helix conformation (15152081). The repeats are arranged head to tail, forming into rods of variable lengths.

ANKRA2 (3SO8) contains five ankyrin repeats at its C-terminus and was initially identified as interaction partner of megalin, which is also known as low-density lipoprotein receptor-related protein 2 (LRP2) (Rader,2000). Megalin is a multiligand-binding receptor located in the plasma membrane of absorptive epithelial cells (11994745). Via yeast two-hybrid screening, ANKRA2 was further characterized as a binding partner of the class IIa histone deacytelases (HDACs) 4 and 5 (Wang,2005, McKinsey,2005). Class IIa HDACs and the cytoplasmic tail of megalin share a conserved PxLPxI/L motif, which mediates specific binding to the ankyrin repeat domain of ANKRA2 (3V20, 3V2X, 3V31). The motif is also found in the alternatively spliced isoform 2 of ABL1. ANKRA2 became also implicated in the 3M syndrome, a primordial growth disorder, since its ankyrin repeat domain interacts with the PxLPxI/L motif located in the C-terminal region of the coiled-coil domain containing 8 (CCDC8) protein (Nie,2015, 4LG6). Nevertheless, the exact biological relevance of ANKRA2 remains elusive.

The ankyrin repeats of ANKRA2 are homologous to those of RFXANK (Krawczyk,2005), sharing 62% sequence identity. RFXANK functions in the positive regulation of major histocompatibility class II (MHC II) gene expression since it associates with RFX5 and RFXAP to form a trimeric transcription factor complex essential for the recruitment of transcriptional co-activators. Mutations in the RFXANK gene cause the immune disorder bare lymphocyte syndrome (BLS). RFXANK is also shown to bind to the transcriptional repressors HDAC4 and HDAC5 (Wang,2005, McKinsey,2005). The functional consequences of RFXANK-HDAC interaction are not well-characterized.

Similar to the ANKRA2 interaction with histone deacetylases and megalin, the ankyrin repeat domain of RFXANK recognizes its binding partners via a consensus PxLPxI/L motif. Therefore, the ankyrin repeat domains of both ANKRA2 and RFXANK bind specifically to PxLPxI/L motifs found in cellular proteins (Xu,2012).

Crystal structure analysis showed that the ankyrin repeat domains of ANKRA2 and RFXANK have a groove with five hydrophobic pockets. The deepest pockets are in the central repeats and they accommodate the core motif residues of the PxLPxI/L motif. A fourth pocket is also occupied by a less conserved hydrophobic residue. The use of hydrophobic pockets along an extended binding groove is likened to a tumbler-lock binding mode (Xu,2012). ANKRA2 and RFXANK bind to PxLPxI/L motif containing proteins with different binding affinities and hence may display specificity for their partners.