The Hydrophilic Loop of Arabidopsis PIN1 Auxin Efflux Carrier Harbors Hallmarks of an Intrinsically Disordered Protein

Auxin transport at the endoplasmic reticulum: roles and structural similarity of PIN-FORMED and PIN-LIKES

Auxin is a crucial plant hormone that controls a multitude of developmental processes. The directional movement of auxin between cells is largely facilitated by canonical PIN-FORMED proteins in the plasma membrane. In contrast, non-canonical PIN-FORMED proteins and PIN-LIKES proteins appear to reside mainly in the endoplasmic reticulum. Despite recent progress in identifying the roles of the endoplasmic reticulum in cellular auxin responses, the transport dynamics of auxin at the endoplasmic reticulum are not well understood. PIN-LIKES are structurally related to PIN-FORMED proteins, and recently published structures of these transporters have provided new insights into PIN-FORMED proteins and PIN-LIKES function. In this review, we summarize current knowledge on PIN-FORMED proteins and PIN-LIKES in intracellular auxin transport. We discuss the physiological properties of the endoplasmic reticulum and the consequences for transport processes across the ER membrane. Finally, we highlight the emerging role of the endoplasmic reticulum in the dynamics of cellular auxin signalling and its impact on plant development.

Substrate recognition and transport mechanism of the PIN-FORMED auxin exporters

Auxins are pivotal plant hormones that regulate plant growth and transmembrane polar auxin transport (PAT) direct patterns of development. The PIN-FORMED (PIN) family of membrane transporters mediate auxin export from the plant cell and play crucial roles in PAT. Here we describe the recently solved structures of PIN transporters, PIN1, PIN3, and PIN8, and also their mechanisms of substrate recognition and transport of auxin. We compare structures of PINs in both inward- and outward-facing conformations, as well as PINs with different binding configurations for auxin. By this comparative analysis, a model emerges for an elevator transport mechanism. Central structural elements necessary for function are identified, and we show that these are shared with other distantly related protein families.

Evolution of the Membrane Transport Protein Domain

Membrane transport proteins are widely present in all living organisms, however, their function, transported substrate, and mechanism of action are unknown. Here we use diverse bioinformatics tools to investigate the evolution of MTPs, analyse domain organisation and loop topology, and study the comparative alignment of modelled 3D structures. Our results suggest a high level of conservancy between MTPs from different taxa on both amino acids and structural levels, which imply some degree of functional similarities. The presence of loop/s of different lengths in various positions suggests tax-on-specific adaptation to transported substrates, intracellular localisation, accessibility for post-translation modifications, and interaction with other proteins. The comparison of modelled structures proposes close relations and a common origin for MTP and Na/H exchanger. Further, a high level of amino acid similarity and identity between archaeal and bacterial MTPs and Na/H exchangers imply conservancy of ion transporting function at least for archaeal and bacterial MTPs.