Site-directed antibodies for probing the structure and biogenesis of phosphatidylinositol glycan-linked membrane proteins: application to placental alkaline phosphatase

Biosynthesis of glycosylphosphatidylinositol (GPI)-anchored membrane proteins in intact cells: specific amino acid requirements adjacent to the site of cleavage and GPI attachment

Mutational studies were previously carried out at the omega site intact cells (Micanovic, R., L. Gerber, J. Berger, K. Kodukula, and S. Udenfriend. 1990. Proc. Natl. Acad. Sci. USA. 87:157-161; Micanovic R., K. Kodukula, L. Gerber, and S. Udenfriend. 1990. Proc. Natl. Acad. Sci. USA: 87:7939-7943) and at the omega + 1 and omega + 2 sites in a cell-free system (Gerber, L., K. Kodukula, and S. Udenfriend. 1992. J. Biol. Chem. 267:12168-12173) of nascent proteins destined to be processed to a glycosylphosphatidyl-inositol (GPI)-anchored form. We have now mutated the omega + 1 and omega + 2 sites in placental alkaline phosphatase (PLAP) cDNA and transfected the wild-type and mutant cDNAs into COS 7 cells. Only glycine at the omega + 2 site yielded enzymatically active GPI membrane-anchored PLAP in amounts comparable to the wild type (alanine). Serine was less active and threonine and valine yielded very low but significant activity. By contrast the omega + 1 site was promiscuous, with only proline being inactive. These and the previous studies indicate that the omega and omega + 2 sites of a nascent protein are key determinants for recognition by COOH-terminal signal transamidase. Comparisons have been made to specific requirements for substitution at the -1, -3 sites of amino terminal signal peptides for recognition by NH2-terminal signal peptidase and the mechanisms of NH2 and COOH-terminal signaling are compared.

Alkaline phosphatase isozymes: recent progress

The past few years have witnessed the reports of significant new events in alkaline phosphatase (AP) isozymes. The cloning of the relevant genes and their nucleotide sequencing have all been accomplished. As a group, the genes for the intestinal, germ cell and placental isozymes have considerable sequence similarity; it is noteworthy that they occupy vicinal positions on chromosome 2, while the tissue unspecific AP gene is located on chromosome 1. The latter makes evolutionary lineage and instances of coordinate expression understandable. Another new development is the demonstration of a phosphatidyl inositol glycan tail on the C-terminus of these chromosome-2 AP genes. This is the major membrane insertion mechanism for AP, which is a cell surface membrane enzyme. This information may be helpful in understanding the phenomenon of the depletion of intestinal mucosal AP during fat absorption. Finally, a discussion has been focussed on recent studies on seminoma and AP, including immunodetection and immunoradiotherapy.

Selectivity of the cleavage/attachment site of phosphatidylinositol-glycan-anchored membrane proteins determined by site-specific mutagenesis at Asp-484 of placental alkaline phosphatase

Many proteins are now known to be anchored to the plasma membrane by a phosphatidylinositol-glycan (PI-G) moiety that is attached to their COOH termini. Placental alkaline phosphatase (PLAP) has been used as a model for investigating mechanisms involved in the COOH-terminal processing of PI-G-tailed proteins. The COOH-terminal domain of pre-pro-PLAP provides a signal for processing during which a largely hydrophobic 29-residue COOH-terminal peptide is removed, and the PI-G moiety is added to the newly exposed Asp-484 terminus. This cleavage/attachment site was subjected to an almost saturation mutagenesis, and the enzymatic activities, COOH-terminal processing, and cellular localizations of the various mutant PLAP forms were determined. Substitution of Asp-484 by glycine, alanine, cysteine, asparagine, or serine (category I) resulted in PI-G-tailed and enzymatically active proteins. However, not all category I mutant proteins were PI-G tailed to the same extent. Pre-pro-PLAP with other substituents at position 484 (threonine, proline, methionine, valine, leucine, tyrosine, tryptophan, lysine, glutamic acid, and glutamine; category II) were expressed, as well as the category I amino acids, but there was little or no processing to the PI-G-tailed form, and this latter group exhibited very low enzyme activity. The bulk of the PLAP protein produced by category II mutants and some produced by category I mutants were sequestered within the cell, apparently in the endoplasmic reticulum (ER). Most likely, certain amino acids at residue 484 are preferred because they yield better substrates for the putative "transamidating" enzyme. In transfected COS cells, at least, posttranslational PI-G-tail processing does not go to completion even for preferred substrates. Apparently PI-G tailing is a requisite for transport from the ER and for PLAP enzyme activity. Proteins that are not transamidated are apparently retained in the ER in an inactive conformation.

Processing at the carboxyl terminus of nascent placental alkaline phosphatase in a cell-free system: evidence for specific cleavage of a signal peptide

Alkaline phosphatase is anchored to the plasma membrane by a carboxyl-terminal phosphatidylinositol glycan moiety. To investigate the biosynthesis of mature alkaline phosphatase, nascent human placental alkaline phosphatase was expressed in a cell-free system and used as substrate for in vitro processing by microsomal extracts. By monitoring the processed product with three site-directed antibodies, it was shown that microsomal extracts from CHO cells that contain other recognized processing activities also remove the carboxyl-terminal signal peptide from the preproenzyme in an apparently selective manner. This peptidase-like cleavage may be brought about by the action of a specific transamidase acting on the nascent protein in the absence of an appropriate phosphatidylinositol glycan cosubstrate.