Flipping the Switch to an Active Spliceosome

The interaction of U1 with the 5Ј splice site has long been known to be required for spliceosome assembly. In fact, according to the sequential assembly model, U1 A spliceosome is a molecular machine that catalyzes is the first spliceosomal component that interacts with the precise excision of an intron from a pre–messenger the substrate RNA and is responsible both for defining RNA (pre-mRNA). A spliceosome rivals a ribosome in the 5Ј intron–exon boundary and for recruiting other its complexity, consisting of five RNA components and components of the spliceosome machine. Staley and up to 50 proteins (for review, see Burge et al., 1999). Guthrie now demonstrate, however, that once the splice-Prior to intron excision, the individual components of a osome is assembled, U1 can have an inhibitory effect spliceosome assemble onto an RNA substrate. In vitro on spliceosome activation. In particular, they created a studies have led to the current model of spliceosome cold-sensitive splicing defect by increasing the length assembly, which depicts a highly ordered sequential of the U1/5Ј splice site pairing, demonstrating that stabi-process resulting in the assembly of a molecular ma-lization of that pairing reduces the efficiency of splicing chine that harbors all of the components necessary for both in vivo and in vitro. Furthermore, spliceosomes with catalysis but is catalytically inactive. Activation of the such a " hyperstabilized " U1/5Ј splice site interaction are assembled spliceosome requires a dramatic reorganiza-apparently locked into an inactive state at the nonper-tion of the assembled components. It is as though the missive temperature; U1 and U4 are not released from assembled spliceosome stands poised to cleave and reconfigure its substrate but remains idle until a " switch is flipped " that activates the machine. This review will focus on two papers in the January issue of Molecular Cell (Kuhn et al., 1999; Staley and Guthrie, 1999) that contribute new insight on the identity, and mechanism of action, of components that " flip the switch " to activate the spliceosome. Figure 1 presents a schematic representation of the transition involved in the conversion of an assembled, inactive spliceosome into a catalytic machine. As shown in Figure 1A, the inactive form of the assembled spliceo-some includes five small nuclear RNAs (snRNAs), U1, U2, U4, U5, and U6. U1 and U2 bind to the substrate by basepairing to the 5Ј splice site (i.e., the boundary between the 5Ј exon and the intron) …