Regulation of CD40 reconstitution into a liposome using different ratios of solubilized LDAO to lipids

Functional reconstitution of Staphylococcus aureus truncated AgrC histidine kinase in a model membrane system

The integral membrane protein AgrC is a histidine kinase whose sensor domains interact with an autoinducing peptide, resulting in a series of downstream responses. In this study, truncated AgrCTM5-6C and AgrCTM5-6C-GFP with GFP as a reporter gene were produced using a bacterial system. Purified AgrCTM5-6C and AgrCTM5-6C-GFP were reconstituted into liposomes by a detergent-mediated method. To achieve high-yield protein incorporation, we investigated the effect of different detergents on protein reconstitution efficiency. The highest incorporation was found with N,N-dimethyldode-cylamine N-oxide during complete liposome solubilization, which resulted in a yield of 85±5%. The COOH-terminus of the protein AgrCTM5-6C was almost exclusively oriented towards the inside of the vesicles. AgrCTM5-6C in proteoliposomes exhibited approximately a 6-fold increase in constitutive activity compared with AgrCTM5-6C in detergent micelles. The reconstitution of AgrCTM5-6C or AgrCTM5-6C-GFP was characterized using dynamic light scattering, fluorescence microscopy, and transmission electron microscopy. Based on the results, the optimal conditions for protein incorporation were defined. These findings contribute to the study of membrane protein structure and function in vitro using a reconstitution system.

Mistic: cellular localization, solution behavior, polymerization, and fibril formation

Mistic represents a family of unique membrane-associating proteins originally found in Bacillus subtilis (M110). As a fusion partner, it has been shown to assist overexpression of foreign integral membrane proteins in E. coli. We have expressed shorter Mistic homologs from other Bacillus species and surprisingly, unlike M110, found them abundant in the cytoplasm. These Mistic homologs including the corresponding shorter sequence (amino acids 27 through 110 of M110) exist as multimeric assemblies in solution in the absence of detergent. Crystals of Mistic from B. leicheniformis (M2) diffracted to 3.2 A resolution, indicating that it exists as a multimer in the crystalline state as well. Moreover, we show that although M2 is mostly alpha-helical, it tends to polymerize and form fibrils. Such oligomerization could potentially mask the charged surface of the monomeric Mistic to assist membrane integration.

Bacterial expression of a eukaryotic membrane protein in fusion to various Mistic orthologs

Mistic, a bacterial membrane-associating protein family, uniquely found in Bacillus species. It enhances expression of eukaryotic membrane proteins at the bacterial membrane. Mistic from B. subtilis (M110), expresses at the Escherichia coli membrane, however its shorter orthologs have been recently shown to be mainly cytoplasmic with varying membrane affinities. Based on that, we hypothesized that the expression level of membrane proteins fused to Mistic is correlated with the degree of membrane association of the particular Mistic protein. We compared expression levels by various Mistic proteins as fusion partners for the Aplysia californica Kv1.1 (aKv1.1) channel as a cargo membrane protein. Mistic from B. atrophaeus (M4), which has the highest membrane association among the shorter orthologs, enhanced expression of the transmembrane domain of aKv1.1 to the highest extent. In contrast, M1, which consists of the 84 C-terminal amino acids of M110 is the most soluble protein and showed the least capacity to express the channel. A chimeric Mistic, constructed with the first alpha-helix (H1) of M110 N-terminally fused to M4, did not increase the level of expression of aKv1.1 beyond those of either the M110 or the M4 fusions. The channel fused to M110, M4 or the aforementioned H1-M4 chimera, expresses in the highest quantity and quality among Mistic proteins, providing suitable sample for structural studies. Our data support the concept that expression levels of 'Misticated' membrane proteins are related to the independent chaperoning character of Mistic via direct membrane association, rather than related to specific sequence-dependent interaction with the E. coli translocon machinery.