NPxY was recognized as the first short signal

NPxY was recognized as the first short signal sequence motif in the cytoplasmic domain of membrane receptors and proteins with critical roles in internalization and trafficking of receptor molecules [130]. Including LDL receptor, beta-1 integrin, megalin, beta-amyloid precursor protein, EGF receptor, and neurotrophin receptor [93,128,130,[141], [142], [143], [144]]. The early studies demonstrated that substitution of a cysteine residue for a tyrosine residue in NPxY (Asn-Pro-x-Tyr) motif of LDL receptor rapidly abolished its endocytosis [145]. Several dileucine-based motifs, including YGLL, SLL, and YWLL are also located in the protein-KHD and GC regions of NPRA, however, their roles in the internalization and trafficking of NPRA is not yet known. Dileucine (LL) motifs regulate internalization and trafficking of several membrane receptors and proteins through the endocytic and secretory pathways [128,[146], [147], [148], [149], [150]]. LL motifs contain 3-7 amino rxr receptor residues in which LL amino acid residues are preceded by a polar amino acids and a negatively charged residue that may be aspartic acid, glutamic acid, or phosphoserine. Dileucine motifs with acidic amino acids are constitutively active and regulate both endocytosis and secretory pathways of membrane receptors and proteins [148]. A short sequence motif Yxxphi is located in the carboxyl-terminal domain of NPRA at residues Y988-x-x-F991; however, its role in the internalization of NPRA remains be determined. These signal motifs with internalization specificity are located within 10-30 amino acid residues from the transmembrane domain of membrane receptors [[151], [152], [153]]. The tyrosine-based Yxxphi sorting signals direct the internalization by interacting with mu1, mu2, mu3, and mu4 subunits of adaptor proteins, including AP-1, AP-2, AP-3, and AP-4, respectively [88,89,139]. The tetrapeptide sequence Yxxphi is usually located in the cytoplasmic domains of the receptors including transferrin and asialoglycoprotein receptors and exhibit critical roles in the endocytosis. In Yxxphi signal motifs, Y represents a tyrosine residue, x is any amino acid residue, and phi represents a residue with large bulky hydrophobic side-chain. Yxxphi signal motifs contain dual specificity with endocytotic functional motif and a trafficking signal within the endosomal and/or secretory pathways [128,134,153,154]. The adaptor protein complexes capture the cargo in the coated vesicles for trafficking of receptors in the subcellular compartments.
Receptor internalization in non-traditional sub-cellular compartments Here, we predict that the alternative fates of activated NPRA are emerging, including trafficking to two other subcellular compartments such as the nucleus and mitochondria (Fig. 5). Comprehensive investigations of receptor trafficking have demonstrated that growth factors induce the internalization of receptor tyrosine kinases (RTKs) and then are either sorted to lysosomes or recycled back on plasma membranes [[155], [156], [157]]. In addition, RTKs traffic through different subcellular compartments such as the nucleus and mitochondria. It has been recognized that trafficking to these novel destinations involves newly identified biochemical and cellular mechanisms and that these trafficking events function in the signal transduction pathways, implicating the receptor itself as a signaling element between the cell surface and the subcellular organelles [158,159]. Similarly, after binding of ligand, cell-surface epidermal growth factor receptor (EGFR) is internalized and trafficked through the endo-lysosomal subcellular compartments or returned to the plasma membrane by recycling mechanisms. Other than these subcellular compartments, EGFR also traffics from the cell surface to the nucleus, Golgi apparatus, endoplasmic reticulum, and mitochondria after endocytosis, where it seems to be involved in multiple biological and physiological functions [160]. Evidence suggest that angiotensin (Ang II) type 1 receptor (AT1R) levels are greatly increased in the mitochondria of aged mice, however, AT2R was localized in mitochondria at a greatest density in young animals and decreased with age [161,162]. Those previous studies indicated that Ang II and AT2R constitute essential components of the mitochondrial inner membrane of various cell types and regulates nitric oxide synthesis to enhance mitochondrial function. These observations also support the hypothesis of intracrine Ang II as previously suggested to illustrate the well –established physiological system [163]. The mechanisms by which these receptors are translocated into the inner membrane of the mitochondria remain unknown. Usually several receptors and proteins localized in mitochondria, are synthesized in the cytoplasm and contain mitochondrial localization sequence at the amino-terminal, which are transported with the help of cytosolic chaperons and delivered to the inner and outer membranes of mitochondria [164,165]. Interestingly, alternative pathways have been suggested for internalization of Ang II ligand-receptor complexes into the subcellular compartments, including microtubule-dependent endocytic pathway [166]. Recent studies have indicated that luminal receptor-bound Ang II is endocytosed in a complex of both AT1R and AT2R as a heterodimer in the endoplasmic reticulum in cultured LLC-PK1 cells [167]. More studies are needed to determine the diversified pathways of the internalization and trafficking of various ligand-receptor complexes in diverse subcellular compartments in both physiological and pathophysiological contexts.