Crystallization is necessary to obtain the three-dimensional structure of proteins and nucleic acids; it often represents the bottleneck in structure determination. Our understanding of crystallization mechanisms is still incomplete. In this review, we emphasize fundamental aspects of the crystallization process. Protein-protein contacts in crystals are complex, involving a delicate balance of specific and nonspecific interactions. Depending on solution conditions, these interactions can lead to nucleation of crystals or to amorphous aggregation; this stage of crystallization has been successfully studied by light scattering. Post-nucleation crystal growth may proceed by mechanisms involving crystal defects or two-dimensional nucleation, as observed by atomic force and interference microscopy. Cessation of growth has been observed but remains incompletely understood. Impurities may play important roles during all stages of crystallization. Phase diagrams can guide optimization of conditions for nucleation and subsequent crystal growth; a theoretical understanding relating these to the intermolecular interactions is beginning to develop.

Multiple diffraction rings appear as a cw laser beam passes through a homeotropic nematic film. The phenomenon is shown to be the result of spatial self-phase modulation that is due to the laser-induced Freedericksz transition.

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Association of the catalytic subunit (C2) with a variety of regulatory subunits is believed to modulate the activity and specificity of protein phosphatase 2A (PP2A). In this study we report the cloning and expression of a new family of B-subunit, the B, associated with the PP2A 0 form. Polymerase chain reactions and cDNA library screening have identified at least seven cDNA isotypes, designated ␣, ␤1, ␤2, ␤3, ␤4, ␥, and ␦. The different ␤ subtypes appear to be generated by alternative splicing. The deduced amino acid sequences of the ␣, ␤2, ␤3, ␤4 and ␥ isoforms predict molecular weights of 57,600, 56,500, 60,900, 52,500, and 68,000, respectively. The proteins are 60 -80% identical and differ mostly at their termini. Two of the isoforms, B␤3 and B␥, contain a bipartite nuclear localization signal in their COOH terminus. No homology was found with other B-or Brelated subunits. Northern analyses indicate a tissuespecific expression of the isoforms. Expression of B␣ protein in Escherichia coli generated a polypeptide of ϳ53 kDa, similar to the size of the B subunit present in the purified PP2A 0 . The recombinant protein was recognized by antibody raised against native B and interacted with the dimeric PP2A (A•C2) to generate a trimeric phosphatase. The deduced amino acid sequences of the B isoforms show significant homology to mammalian, fungal, and plant nucleotide sequences of unknown function present in the data bases. Notably, a high degree of homology (55-66%) was found with a yeast gene, RTS1, encoding a multicopy suppressor of a rox3 mutant. Our data indicate that at least seven B subunit isoforms may participate in the generation of a large number of PP2A 0 holoenzymes that may be spatially and/or functionally targeted to different cellular processes.Protein phosphatase 2A (PP2A) 1 is one of the major serine/ threonine protein phosphatases present in the cell and is involved in the control of many cellular functions and metabolic pathways (reviewed by Cohen (1989), Mumby and Walter (1993), DePaoli-Roach et al. (1994), and Mayer-Jaekel and Hemmings (1994)). The Ser/Thr protein phosphatases, with the exception of PP2C, consist of multimeric structures. Their catalytic subunit associates with specific proteins, which serve as targeting/regulatory subunits and play substantial roles in the control of phosphatase activity.PP2A is a family of holoenzymes containing a common core of a 36-kDa catalytic (C2) subunit and a 63-kDa A subunit associated with a variety of regulatory B-subunits (B, BЈ, and BЉ) to form the trimeric PP2A 1 , PP2A 0 , and polycation-stimulated protein phosphatase M, respectively (Tung et al., 1985;Waelkens et al., 1987;Mumby et al., 1987;Zolnierowicz et al., 1994). Takeda and co-workers (Usui et al., 1988) also isolated from human erythrocytes a PP2A form that contained a polypeptide of 74 kDa associated with the A⅐C2 core. Molecular cloning has identified in mammals two isoforms each of the C2 (da Cruz e Silva and Cohen, 1987;Green et al., 1987;Stone et al., 1987) and the A (Walter et al....

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