Assembling the yeast LSM protein complex

Many cellular functions are meditated not by single proteins but, rather, by macromolecular complexes containing multiple subunits often with specific functions. The determination of structures of large hetrogenous protein complexes poses particular difficulty for structural biologists, requiring the careful assembly of components into a viable complex in a state suitable for data collection. Avenues for in vitro assembly can involve recombinant construction of all or some components via coexpression, or expression of polyprotiens with engineered linkers. We are studying the organisation of LSm proteins, known to organize as multiprotien ring assemblies which interact with RNAs to allow chemical modification or nuclease protection. The precise biological function of these assemblies is determined by their constituent proteins. Thus heptameric LSm[1-7] assists cytoplasmic mRNA degradation, whereas when LSm8 replaces LSm1, the complex instead promotes nuclear biogenesis of the U6 snRNP. We are reconstituting mixed complexes of LSms with the aim of defining the heptamers LSm[1-7] and LSm[2-8] by crystallography. We describe our polyprotien construction of appropriate dimer and trimer combinations to prepare stable mixed LSm subcomplexes.