Fragment antigen binding domains (F_(ab)s) as tools to study assembly-line polyketide synthases

The crystallization of proteins remains a bottleneck in our fundamental understanding of their functions.Therefore,discovering tools that aid crystallization is crucial.In this review,the versatility of fragment-antigen binding domains(F_(ab)s)as protein crystallization chaperones is discussed.F_(ab)s have aided the crystallization of membrane-bound and soluble proteins as well as RNA.The ability to bind three F_(ab)s onto a single protein target has demonstrated their potential for crystallization of challenging proteins.We describe a high-throughput workflow for identifying F_(ab)s to aid the crystallization of a protein of interest(POI)by leveraging phage display technologies and differential scanning fluorimetry(DSF).This workflow has proven to be especially effective in our structural studies of assembly-line polyketide synthases(PKSs),which harbor flexible domains and assume transient conformations.PKSs are of interest to us due to their ability to synthesize an unusually broad range of medicinally relevant compounds.Despite years of research studying these megasynthases,their overall topology has remained elusive.One F ab in particular,1B2,has successfully enabled X-ray crystallographic and single particle cryo-electron microscopic(cryoEM)analyses of multiple modules from distinct assembly-line PKSs.Its use has not only facilitated multidomain protein crystallization but has also enhanced particle quality via cryoEM,thereby enabling the visualization of intact PKS modules at near-atomic(3–5Å)resolution.The identification of PKS-binding F_(ab)s can be expected to continue playing a key role in furthering our knowledge of polyketide biosynthesis on assembly-line PKSs.

Identification of common and distinct origins of human serum and breastmilk IgA1 by mass spectrometry-based clonal profiling

The most abundant immunoglobulin present in the human body is IgA. It has the highest concentrations at the mucosal lining and in biofluids such as milk and is the second most abundant class of antibodies in serum. We assessed the structural diversity and clonal repertoire of IgA1-containing molecular assemblies longitudinally in human serum and milk from three donors using a mass spectrometry-based approach. IgA-containing molecules purified from serum or milk were assessed by the release and subsequent analysis of their Fab fragments. Our data revealed that serum IgA1 consists of two distinct structural populations, namely monomeric IgA1 (∼80%) and dimeric joining (J-) chain coupled IgA1 (∼20%). Also, we confirmed that IgA1 in milk is present solely as secretory (S)IgA, consisting of two (∼50%), three (∼33%) or four (∼17%) IgA1 molecules assembled with a J-chain and secretory component (SC). Interestingly, the serum and milk IgA1-Fab repertoires were distinct between monomeric, and J-chain coupled dimeric IgA1. The serum dimeric J-chain coupled IgA1 repertoire contained several abundant clones also observed in the milk IgA1 repertoire. The latter repertoire had little to no overlap with the serum monomeric IgA1 repertoire. This suggests that human IgA1s have (at least) two distinct origins;one of these produces dimeric J-chain coupled IgA1 molecules, shared in human serum and milk, and another produces monomeric IgA1 ending up exclusively in serum.