Involvement of N-myristoylation in monoclonal antibody recognition sites on chimeric G protein alpha subunits

Myristic acid increases the activity of dihydroceramide Delta4-desaturase 1 through its N-terminal myristoylation

Dihydroceramide Delta4-desaturase (DES) catalyzes the desaturation of dihydroceramide into ceramide. In mammals, two gene isoforms named DES1 and DES2 have recently been identified. The regulation of these enzymes is still poorly understood. This study was designed to examine the possible N-myristoylation of DES1 and DES2 and the effect of this co-translational modification on dihydroceramide Delta4-desaturase activity. N-MyristoylTransferases (NMT) catalyze indeed the formation of a covalent linkage between myristoyl-CoA and the N-terminal glycine of candidate proteins, as found in the sequence of DES proteins. The expression of both rat DES in COS-7 cells evidenced first that DES1 but not DES2 was associated with an increased dihydroceramide Delta4-desaturase activity. Then, we showed that recombinant DES1 was myristoylated in vivo when expressed in COS-7 cells. In addition, in vitro myristoylation assay with a peptide substrate corresponding to the N-terminal sequence of the protein confirmed that NMT1 has a high affinity for DES1 myristoylation motif (apparent K(m)=3.92 microM). Compared to an unmyristoylable mutant form of DES1 (Gly replaced by an Ala), the dihydroceramide Delta4-desaturase activity of the myristoylable DES1-Gly was reproducibly and significantly higher. Finally, the activity of wild-type DES1 was also linearly increased in the presence of increased concentrations of myristic acid incubated with the cells. These results demonstrate that DES1 is a newly discovered myristoylated protein. This N-terminal modification has a great impact on dihydroceramide Delta4-desaturase activity. These results suggest therefore that myristic acid may play an important role in the biosynthesis of ceramide and in sphingolipid metabolism.

Functional characterization of a series of mutant G protein alphaq subunits displaying promiscuous receptor coupling properties

The N termini of two G protein alpha subunits, alphaq and alpha11, differ from those of other alpha subunits in that they display a unique, highly conserved six-amino acid extension (MTLESI(M)). We recently showed that an alphaq deletion mutant lacking these six amino acids (in contrast to wild type alphaq) was able to couple to several different Gs- and Gi/o-coupled receptors, apparently due to promiscuous receptor/G protein coupling (Kostenis, E., Degtyarev, M. Y., Conklin, B. R., and Wess, J. (1997) J. Biol. Chem. 272, 19107-19110). To study which specific amino acids within the N-terminal segment of alphaq/11 are critical for constraining the receptor coupling selectivity of these subunits, this region of alphaq was subjected to systematic deletion and alanine scanning mutagenesis. All mutant alphaq constructs (or wild type alphaq as a control) were coexpressed (in COS-7 cells) with the m2 muscarinic or the D2 dopamine receptors, two prototypical Gi/o-coupled receptors, and ligand-induced increases in inositol phosphate production were determined as a measure of G protein activation. Surprisingly, all 14 mutant G proteins studied (but not wild type alphaq) gained the ability to productively interact with the two Gi/o-linked receptors. Similar results were obtained when we examined the ability of selected mutant alphaq subunits to couple to the Gs-coupled beta2-adrenergic receptor. Additional experiments indicated that the functional promiscuity displayed by all investigated mutant alphaq constructs was not due to overexpression (as compared with wild type alphaq), lack of palmitoylation, or initiation of translation at a downstream ATG codon (codon seven). These data are consistent with the notion that the six-amino acid extension characteristic for alphaq/11 subunits forms a tightly folded protein subdomain that is critical for regulating the receptor coupling selectivity of these subunits.

Role of subunit diversity in signaling by heterotrimeric G proteins

The heterotrimeric G proteins are extensively involved in the regulation of cells by extracellular signals. The receptors that control them are often the targets of drugs. There are many isoforms of each of the three subunits that make up these proteins. Thus far, genes for at least sixteen alpha subunits, five beta subunits, and eleven gamma subunits have been identified. In addition, some of these proteins have splice variants or are differentially modified. Based upon what is already known, there are well over a thousand possible G protein heterotrimer combinations. The role of subunit diversity in heterotrimer formation and its effect on signaling by G proteins are still not well understood. However, many current lines of research are leading toward an understanding of these roles. The functional significance of subunit heterogeneity is related to the mechanisms used by G proteins to transmit and integrate the many signals coming into cells through this system. Described here are the basic mechanisms by which G proteins integrate cellular responses, the possible role of subunit heterogeneity in these mechanisms, and the evidence for and against their physiological significance. Recent studies suggest the likely possibility that subunit heterogeneity plays an important role in signaling by G proteins. This role has the potential to extend substantially the flexibility of G proteins in mediating cellular responses to extracellular signals. However, the details of this are yet to be worked out, and they are the subject of many different avenues of research.

Myristoylation

N-myristoylation is an acylation process absolutely specific to the N-terminal amino acid glycine in proteins. This maturation process concerns about a hundred proteins in lower and higher eukaryotes involved in oncogenesis, in secondary cellular signalling, in infectivity of retroviruses and, marginally, of other virus types. Thy cytosolic enzyme responsible for this activity, N-myristoyltransferase (NMT), studied since 1987, has been purified from different sources. However, the studies of the specificities of the various NMTs have not progressed in detail except for those relating to the yeast cytosolic enzyme. Still to be explained are differences in species specificity and between various putative isoenzymes, also whether the data obtained from the yeast enzyme can be transposed to other NMTs. The present review discusses data on the various addressing processes subsequent to myristoylation, a patchwork of pathways that suggests myristoylation is only the first step of the mechanisms by which a protein associates with the membrane. Concerning the enzyme itself, there are evidences that NMT is also present in the endoplasmic reticulum and that its substrate specificity is different from that of the cytosolic enzyme(s). These differences have major implications for their differential inhibition and for their respective roles in several pathologies. For instance, the NMTs from mammalians are clearly different from those found in several microorganisms, which raises the question whether the NMT may be a new targets for fungicides. Finally, since myristoylation has a central role in virus maturation and oncogenesis, specific NMT inhibitors might lead to potent antivirus and anticancer agents.

Role of physiology in the determination of protein heterogeneity

One of the major problems faced in the manufacture of recombinant proteins is the assessment and control of product heterogeneity, which is caused mainly by the biological variation in post-translational processing. The past year has witnessed significant improvements both in analytical methods for detecting variation in this processing and in our ability to control it.

Heterotrimeric G proteins

Over the past year, the thrust of work in the field of heterotrimeric G proteins has been primarily in the following areas: first, resolution of their three-dimensional structures by X-ray crystallography; second, elucidation of the effect of lipid modifications on the Galpha and Ggamma subunits; third, understanding the role of the Gbetagamma dimer in stimulation of a variety of effectors following receptor activation; and fourth, identification of the points of contact among the Galpha, Gbeta, and Ggamma subunits, and between these subunits and their cognate receptor or effector molecules.

Hydrophobicity and subunit interactions of rod outer segment proteins investigated using Triton X-114 phase partitioning

Triton X-114 phase partitioning, a procedure used for purifying integral membrane proteins, was used to study protein components of the mammalian visual transduction cascade. An integral membrane protein, rhodopsin, and two isoprenylated protein complexes, cyclic GMP phosphodiesterase and Gt beta gamma, partitioned into the detergent-rich phase. Arrestin, a soluble protein, accumulated in the aqueous phase. Gt alpha distributed about equally between phases whether GDP (Gt alpha.GDP) or GTP (Gt alpha.GTP) was bound. Gt beta gamma increased recovery of Gt alpha.GDP but not Gt alpha.GTP in the detergent phase. Trypsin-treated Gt alpha, which lacks the fatty acylated amino-terminal 2-kDa region, accumulated to a greater extent in the aqueous phase than did intact Gt alpha. Trypsinized cGMP phosphodiesterase, which lacks the isoprenyl group, partitioned into the aqueous phase. A carboxyl-terminal truncated mutant (Val-331 stop) of Gt alpha accumulated more in the aqueous phase then did recombinant full-length Gt alpha, supporting the role of the carboxyl terminus in increasing its hydrophobicity. N-Myristoylated recombinant Go alpha was more hydrophobic than recombinant Go alpha without myristate. ADP-ribosylation of Gt alpha catalyzed by NAD:arginine ADP-ribosyltransferase, but not by pertussis toxin, increased hydrophilicity. Triton X-114 phase partitioning can thus semiquantify the hydrophobic nature of proteins and protein domains. It may aid in evaluating changes associated with post-translational protein modification and protein-protein interactions in a defined system.