Sparse matrix screens

Both methods require the use of a broad range of crystallization solutions for the initial screening of crystals. These screens usually come in two types, grid screens and sparse matrix screens ... 

Sparse matrix screens These screens are based on the factorial or incomplete factorial statistical approach for designing the screening experiment (Jancarik and Kim, 1991). An extended version of this approach, called sparse matrix sampling has been developed to cover a very large number of conditions for initial screening. 

Fig. 11.10 Histogram of c7stallization hits for sparse matrix screens of model proteins. Number of screens tested on each protein are lysosyme (L)=2, glucose isomer-ase (Cl)=2, protease K (PK) = 1, bovine liver catalase (BLC) = 1, xylanase (X) = 2, bacterial primase catalytic core domain (BPC) = 3, bovine pancreas t7psin (BT) = 1, thaumatin (T) = l, mycobacterial RNase (MR)=3.

Jancarik J, Kim SH. 1991. Sparse matrix sampling a screening method for crystallization of proteins. J Appl Cryst 24 409-411. 

Crystallization additives are a common method to improve crystal quality in proteins (McPherson and Cudney, 2006). With RNA, it is common to screen a series of different cations, as their role in RNA structure and catalysis is well documented for many RNA and RNA-protein systems (Pyle, 2002). Since various cations will interact with the RNA differently, our lab uses a cation screen comprised simple metal cations and polyamines (Table 6.1). Each solution in this table is a lOx stock that is added to an optimized condition (or one found in a sparse matrix) and the set of 24 conditions assessed for potential improvement in crystal quality. 

During these trials, the chip also produced crystals of two targets that had not been seen by conventional screening. A previously unidentified crystal form of the bacterial 70S ribosome was obtained in three conditions of a sparse matrix of precipitants (Hampton Crystal Screen I), demonstrating that large protein-nudeic acid complexes may be crystallized in chip (C. Hansen, A. Vila-Sanjurjo and J. Cate, personal communication). Crystals of a previously uncrystallized mycobacterial RNase were also obtained from a single experimental condition on chip, whereas no crystals had been observed for this sample despite prior extensive... 

Several crystallization-screening protocols have been developed over the past decade to help identify initial crystallization conditions. These include footprint screening (Sutra et al., 1992), sparse matrix sampling (Jancarik and Kim, 1991), statistical methods (Carter and Carter, 1979), and MON-48 (Shieh et al., 1991). Reagent kits for some of these screens are now commercially available (Hampton Research [www.hamptonresearch.com], Emerald BioStructures [www.emeraldbio-structures.com]) and are commonly used in the laboratory. 

---