GAP-43 has been termed a "growth" or "plasticity" protein because it is expressed at high levels in neuronal growth cones during development and during axonal regeneration. By homologous recombination, we generated mice lacking GAP-43. The mice die in the early postnatal period. GAP-43-deficient retinal axons remain trapped in the chiasm for 6 days, unable to navigate past this midline decision point. Over the subsequent weeks of life, most GAP-43-deficient axons do enter the appropriate tracts, and the adult CNS is grossly normal. There is no evidence for interference with nerve growth rate, and cultured neurons extend neurites and growth cones in a fashion indistinguishable from controls. Thus, the GAP-43 protein is not essential for axonal outgrowth or growth cone formation per se, but is required at certain decision points, such as the optic chiasm. This is compatible with the hypothesis that GAP-43 serves to amplify pathfinding signals from the growth cone.

Heterotrimeric G proteins, composed of G␣ and G␤␥ subunits, transmit signals from cell surface receptors to cellular effector enzymes and ion channels. The G␣ o protein is the most abundant G␣ subtype in the nervous system, but it is also found in the heart. Its function is not completely known, although it is required for regulation of N-type Ca 2؉ channels in GH 3 cells and also interacts with GAP43, a major protein in growth cones, suggesting a role in neuronal pathfinding. To analyze the function of G␣ o , we have generated mice lacking both isoforms of G␣ o by homologous recombination. Surprisingly, the nervous system is grossly intact, despite the fact that G␣ o makes up 0.2-0.5% of brain particulate protein and 10% of the growth cone membrane. The G␣ o ؊͞؊ mice do suffer tremors and occasional seizures, but there is no obvious histologic abnormality in the nervous system. In contrast, G␣ o ؊͞؊ mice have a clear and specific defect in ion channel regulation in the heart. Normal muscarinic regulation of L-type calcium channels in ventricular myocytes is absent in the mutant mice. The L-type calcium channel responds normally to isoproterenol, but there is no evident muscarinic inhibition. Muscarinic regulation of atrial K ؉ channels is normal, as is the electrocardiogram. The levels of other G␣ subunits (G␣ s , G␣ q , and G␣ i ) are unchanged in the hearts of G␣ o ؊͞؊ mice, but the amount of G␤␥ is decreased. Whichever subunit, G␣ o or G␤␥, carries the signal forward, these studies show that muscarinic inhibition of L-type Ca 2؉ channels requires coupling of the muscarinic receptor to G␣ o . Other cardiac G␣ subunits cannot substitute.Heterotrimeric G proteins, composed of G␣ and G␤␥ subunits, transmit signals from cell surface receptors to cellular effector enzymes and ion channels. One type of G␣ subunit, G␣ o , is extremely abundant in the brain, where it was first identified (1, 2), but it is also expressed in heart, pituitary, and pancreas. In addition to G␣ o , both the brain and the heart contain other closely related G␣ subunits (for example, members of the G␣ i group that are, like G␣ o , substrates for ADP ribosylation by pertussis toxin), as well as G␣ s (which stimulates adenylyl cyclase) and G␣ q (which stimulates phospholipase C␤).The exact function of G␣ o in heart and brain is not known. It is an extremely abundant protein in the nervous system, making up 0.2-0.5% of brain particulate protein (3, 4) and 10% of the growth cone membrane (5). In the nervous system, G␣ o has been postulated to play several roles. The ability of G␣ o to bind GTP␥S can be modulated by GAP43 (neuromodulin), an abundant growth cone protein that is important for neuronal pathfinding (5). Potentially, G␣ o could be part of the signaling cascade that regulates neuronal guidance. Its appearance in the mouse central nervous system is consistent with such a role, since it begins to appear as neurons terminally differentiate and increases as they send out processes (6). The G␣ o protein is conserved in Drosophila, w...

We recently described a 125 kd membrane glycoprotein in Saccharomyces cerevisiae which is anchored in the lipid bilayer by an inositol‐containing phospholipid. We now find that when S. cerevisiae cells are metabolically labeled with [3H]myoinositol, many glycoproteins become labeled more strongly than the 125 kd protein. Myoinositol is attached to these glycoproteins as part of a phospholipid moiety which resembles glycophospholipid anchors of other organisms. Labeling of proteins with [3H]myoinositol for short times and in secretion mutants blocked at various stages of the secretory pathway shows that these phospholipid moieties can be added to proteins in the endoplasmic reticulum and that these proteins are transported to the Golgi by the regular secretory pathway. sec53, a mutant which cannot produce GDP‐mannose at 37 degrees C, does not incorporate myoinositol or palmitic acid into membrane glycoproteins at this temperature, suggesting that GDP‐mannose is required for the biosynthesis of these phospholipid moieties. All other secretion and glycosylation mutants tested add phospholipid moieties to proteins normally.