M O N I T O R

Modulation of serotonin-controlled behaviors by G O in Caenorbabditis

elegans

L. SI~GALAT, D.A. ELKES AND J.M. KAPLAN

Science267, 1648-1651

Participation of the protein G O in multiple aspects of behavior in

C. elegans

J.E. MENDEL E T A L

Science 267, 1652-1655

In the mind of a worm, serotonin means contentment. Without it, the worm becomes dissatisfied and searches nervously for greener pastures. S4galat et al., hoping to find out what serotonin does in the worm's mind, identified mutants that acted nervously even when given serotonin. Meanwhile Mendel et al. set out to learn what the G protein G o does. They cloned the C. elegans gene (goa-1) encoding the subunit Gc% and knocked it out. Both groups discovered that worms lacking G% are nervous. Increasing the goa-1 dosage, or locking G% in the signalling state by inactivating the GTPase, had the opposite effect: the worms became torpid. Thus, the effect of serotonin on locomotion may be mediated in part by G%. The expression of goa-1 is not limited to the neurons that control loco- motion; it is expressed in all neurons and in many of the muscles they con- trol. As might be expected from its broad expression, the goa-1 mutation affected several behaviours other than locomotion: feeding, egg-laying, def- ecation and mating. Each of these behaviours is affected by serotonin, as well. Does Go% mediate all the effects of serotonin? It may be so for defecation and mating, but feeding and egg-laying seem not to be so simple. Feeding is increased in the presence of serotonin or excess Ge%. However, worms lacking Ge% still increase feeding in response to serotonin. The effects of serotonin on egg-laying are opposite to those of GoG; serotonin stimulates egg- laying, but G~x o inhibits it. S4galat et al. suggest that serotonin acts through G% in the egg-laying muscles by causing the release of G[3",/, but this explanation is difficult to reconcile with the obser- vation of Mendel et al. that GTPase- minus G% inhibits egg-laying. In summary, these papers are the start of something interesting. They begin to answer some questions, but they pose many new ones. Through what effec- tors does Go G act? Why is serotonin not coupled to G% in some behaviours, or coupled differently? What role do G proteins other than G o play in the modulation of behaviour by acetyl- choline, dopamine or GABA? /~

Correlation of terminal cell cycle arrest of skeletal muscle with

induction of p21 by myoD

O. HALEVY ETAL.

Science 267, 1018-1021

Inhibition of myogenic differentiation in proliferating myoblasts by cyclin

Dl-dependent kinase

S.X. SKAPEK, J. RHEE, D.B. SP1CER AND A.B. LASSAR

Science 267, 1022-1024

p53-independent expression of p21Ctp 1 in muscle and other terminally

differentiating cells

S.B. PARKER ETAL.

Science 267, 1024-1027

Dividing cells that make the commit- ment to differentiate invariably undergo a withdrawal from the cell cycle. Clues to understanding this intimate connec- tion of events have now come with the observation that cell-cycle arrest in terminally differentiating skeletal muscle cells correlates with expression of an inhibitor (p21) of cyclin-dependent kinases (cdks). Using in situ hybridiza- tion, Parker et al. showed that p21 expression coincides with muscle differ- entiation in the mouse embryo, comp- lementing an observation by Halevy et aL that p21 is induced during differen- tiation of muscle cells in vitro. The 10T1/2 fibroblasts only express p21 if they have been infected with a retrovirus encoding MyoD, suggesting MyoD may directly induce p21. This possibility is

supported by the observations that MyoD-dependent expression of p21 slightly precedes expression of muscle proteins and that MyoD can cause cell- cycle arrest in certain cell types that cannot be induced to express any musde differentiation markers. However, although MyoD is sufficient to induce p21, Parker et aL note that p21 expression in vivo generally precedes that of MyoD and can occur in mouse strains deleted for both MyoD and myogenin. This discrepancy may be explained by func- tional redundancy within the complex network of muscle-specific transcrip- tional regulators. Skapek et al., inves- tigating proliferating myoblasts, show that expression of cyclin D1 specifically inhibits MyoD-induced expression of a reporter gene and promotes hyper- phosphorylation of MyoD. Cyclin D1 levels are high in proliferating myo- blasts but decrease upon differ- entiation, p21 reverses the cyclin D1 inhibition of MyoD, indicating that cyclin D1 inhibits MyoD by activating a cdk. Previous work has shown that cell-cycle arrest in differentiated muscle cells requires the retinoblastoma tumour suppressor protein (Rb), which is negatively regulated by cdks. Halevy et al. suggest that MyoD activates Rb by an indirect mechanism, perhaps via induction of p21 that in turn inhibits cdks as the cells differentiate. Finally, induction of p21 by MyoD is indepen- dent of p53, a protein that induces p21 in response to DNA damage. This raises the interesting possibility that p21 can be induced in response to a range of different signals to bring about cell-cycle arrest. /~

The DNA-binding domain of the initiator protein DnaA

A. ROTH AND W. MESSER

EMBOJ. 14, 2106-2111

Magnetic beads are versatile com- ponents of the modern biologist's toolkit. Roth and Messer add to their applicability by demonstrating their use for identifying an apparently novel DNA binding domain in the highly con- served bacterial replication initiation protein, DnaA. This 467amino acid protein plays a key role in the normal initiation of DNA replication at or/C, by binding to repeated 9bp motifs known as 'Dna boxes', it mediates a local unwinding of an AT-rich region, thus directing the other components required for replication initiation to oriC DnaA is also required for primo- some assembly and can mediate tran- scriptional activation, repression and

TIG JULY 1995 VOL. 11 No. 7

2 6 2

termination by binding to DnaA boxes in other genes. Fusion proteins of DnaA or DnaA fragments were attached to streptavidin-coated magnetic beads in one of two ways using biotin as a link- ing ligand. Specific DNA binding was then assayed by capture of a radioactive target DNA. In principle, this method can also be used to isolate the DNA target of a specific DNA-binding protein. The 94 C-terminal amino acids of DNA were shown to be sufficient for specific DNA binding and contain three of the 'classical' d n a ts mutations. Inspection of the amino acid sequence of this region does not reveal an obvious DNA-binding motif, although three potential c~ helices are present. During analysis of this sequence, similarities between DnaA and transcriptional activators of the NtrC family were observed; this is par- ticularly interesting in view of the fact that both proteins direct local DNA unwinding as part of their function. /~