Several peptide fragments are produced by proteolytic cleavage of the opioid peptide precursor proenkephalin A, and among these are a number of enkephalin fragments, in particular bovine adrenal medulla peptide 22 (BAM22). These peptide products have been implicated in diverse biological functions, including analgesia. We have cloned a newly identified family of 'orphan' G protein--coupled receptors (GPCRs) and demonstrate that BAM22 and a number of its fragments bind to and activate these receptors with nanomolar affinities. This family of GPCRs is uniquely localized in the human and rat small sensory neuron, and we called this family the sensory neuron--specific G protein--coupled receptors (SNSRs). Receptors of the SNSR family are distinct from the traditional opioid receptors in their insensitivity to the classical opioid antagonist naloxone and poor activation by opioid ligands. The unique localization of SNSRs and their activation by proenkephalin A peptide fragments indicate a possible function for SNSRs in sensory neuron regulation and in the modulation of nociception.

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Phosphatases and kinases are the cellular signal transduction enzymes that control protein phosphorylation. PRL phosphatases constitute a novel class of small (20 kDa), prenylated phosphatases with oncogenic activity. In particular, PRL-3 is consistently overexpressed in liver metastasis in colorectal cancer cells and represents a new therapeutic target. Here, we present the solution structure of PRL-3, the first structure of a PRL phosphatase. The structure places PRL phosphatases in the class of dual specificity phosphatases with closest structural homology to the VHR phosphatase. The structure, coupled with kinetic studies of site-directed mutants, identifies functionally important residues and reveals unique features, differentiating PRLs from other phosphatases. These differences include an unusually hydrophobic active site without the catalytically important serine/threonine found in most other phosphatases. The position of the general acid loop indicates the presence of conformational change upon catalysis. The studies also identify a potential regulatory role of Cys 49 that forms an intramolecular disulfide bond with the catalytic Cys 104 even under mildly reducing conditions. Molecular modeling of the highly homologous PRL-1 and PRL-2 phosphatases revealed unique surface elements that are potentially important for specificity.PRL (for phosphatase of regenerating liver) phosphatases constitute a novel class of small tyrosine phosphatases involved in the modulation of cell growth. Initial studies identified PRL-1 as an intermediate-early gene expressed in the early response of regenerating liver tissue to mitogens (1). Overexpression of this protein was shown to lead to cellular transformation (1-3). The biological role of PRL-1 is tissue-dependent. Its overexpression is associated with cell proliferation in the liver (1) but with differentiation of epithelial cells in the digestive system (4). The closely related phosphatases PRL-2 and PRL-3 are also involved in growth regulation, proliferation, and cell invasion (3, 5, 6). All three proteins are prenylated at their C terminus, which critically affects their cellular localization and function (6 -8). As shown for the human PRL-2, the role of PRLs is associated with the regulation of progression through mitosis, and their cellular localization is likely controlled by the cell cycle (8).PRL phosphatases are widely distributed in eukaryotes. In humans, PRL-1 and PRL-2 are ubiquitously expressed in various tissues (6), whereas PRL-3 is normally expressed in cardiac and skeletal muscles (5). Comprising typically only 140 -180 amino acids, PRLs are among the smallest phosphatases. They consist of a single catalytic domain lacking any auxiliary docking/regulatory domains other than the prenylation site at the C terminus. PRLs contain the protein-tyrosine phosphatase (PTPase) 1 active consensus motif HCXXGXXR, referred to as the P-loop; however, their primary sequence shows only remote similarity to phosphatases in other regions. Tyrosine-specific phosphatases a...

In vertebrates all iron is taken up via the carrier protein transferrin. The carrier first binds its receptor and the receptor-ligand complex is then internalized via coated pits. The transferrin receptor is a transmembrane glycoprotein (apparent molecular weight (MW) 180,000) composed of two disulphide-bonded sub-units (each of apparent MW 90,000) It contains three N-linked glycan units and is post-translationally modified with both phosphate and fatty acyl groups. Here we have determined the nucleotide sequence of the coding region of the human transferrin receptor mRNA and from this deduced the amino acid sequence of the protein. The receptor does not contain an N-terminal signal peptide but there is a membrane-spanning segment 62 amino acids from the N-terminus. It therefore has a somewhat unusual configuration with a small N-terminal cytoplasmic domain and a C-terminal extracellular domain of 672 amino acids.

The screening of a size-selected cDNA library from the ovary revealed the existence of a second form of PRL receptor in the rat. The polypeptide sequence deduced from cDNAs has a much longer cytoplasmic domain (357 amino acids) than the form previously identified in the liver (57 amino acids). Nucleotide sequence analysis and comparison with rabbit, mouse, and human PRL receptor cDNAs suggests that the two forms of rat PRL receptor result from alternative splicing of a primary transcript. Complementary DNAs encoding the long form of the receptor were also found in a library prepared from estradiol-treated rat liver, although they represent a minor fraction of total PRL receptor cDNAs obtained from this tissue. DNA polymerase chain reaction amplification of cDNA confirmed the presence of the two receptor forms in both the ovary and liver. Northern analysis, using probes that specifically hybridize with either form of mRNA, indicates a major transcript of 1.8 kilobases (kb) in estradiol-treated liver, which encodes the receptor with a short cytoplasmic domain, while the long form of the receptor is encoded by mRNAs of 2.5 and 3 kb. In the ovary, a complex pattern of hybridization to multiple mRNAs (1.8-5.5 kb) is obtained with the probe specific to the long form, and essentially only a 5.5-kb mRNA is obtained with the probe specific to the short form. The predicted size of the mature form of the long PRL receptor (PRL-R2) is 591 amino acid residues.(ABSTRACT TRUNCATED AT 250 WORDS)