SUMMARYMultistep phosphorelay (MSP) signaling mediates responses to a variety of important stimuli in plants. In Arabidopsis MSP, the signal is transferred from sensor histidine kinase (HK) via histidine phosphotransfer proteins (AHP1-AHP5) to nuclear response regulators. In contrast to ancestral two-component signaling in bacteria, protein interactions in plant MSP are supposed to be rather nonspecific. Here, we show that the C-terminal receiver domain of HK CKI1 (CKI1 RD ) is responsible for the recognition of CKI1 downstream signaling partners, and specifically interacts with AHP2, AHP3 and AHP5 with different affinities. We studied the effects of Mg 2+ , the co-factor necessary for signal transduction via MSP, and phosphorylation-mimicking BeF 3 ) on CKI1 RD in solution, and determined the crystal structure of free CKI1 RD and CKI1 RD in a complex with Mg 2+ . We found that the structure of CKI1 RD shares similarities with the only known structure of plant HK, ETR1 RD , with the main differences being in loop L3. Magnesium binding induces the rearrangement of some residues around the active site of CKI1 RD , as was determined by both X-ray crystallography and NMR spectroscopy. Collectively, these results provide initial insights into the nature of molecular mechanisms determining the specificity of MSP signaling and MSP catalysis in plants.

The multistep phosphorelay (MSP) is a central signaling pathway in plants integrating a wide spectrum of hormonal and environmental inputs and controlling numerous developmental adaptations. For the thorough comprehension of the molecular mechanisms underlying the MSP-mediated signal recognition and transduction, the detailed structural characterization of individual members of the pathway is critical. In this review we describe and discuss the recently known crystal and nuclear magnetic resonance structures of proteins acting in MSP signaling in higher plants, focusing particularly on cytokinin and ethylene signaling in Arabidopsis thaliana. We discuss the range of functional aspects of available structural information including determination of ligand specificity, activation of the receptor via its autophosphorylation, and downstream signal transduction through the phosphorelay. We compare the plant structures with their bacterial counterparts and show that although the overall similarity is high, the differences in structural details are frequent and functionally important. Finally, we discuss emerging knowledge on molecular recognition mechanisms in the MSP, and mention the latest findings regarding structural determinants of signaling specificity in the Arabidopsis MSP that could serve as a general model of this pathway in all higher plants.

The spontaneous host-range mutants 812F1 and K1/420 are derived from polyvalent phage 812 that is almost identical to phage K, belonging to family Myoviridae and genus Kayvirus. Phage K1/420 is used for the phage therapy of staphylococcal infections. Endolysin of these mutants designated LysF1, consisting of an N-terminal cysteine-histidine-dependent aminohydrolase/peptidase (CHAP) domain and C-terminal SH3b cell wall-binding domain, has deleted middle amidase domain compared to wild-type endolysin. In this work, LysF1 and both its domains were prepared as recombinant proteins and their function was analyzed. LysF1 had an antimicrobial effect on 31 Staphylococcus species of the 43 tested. SH3b domain influenced antimicrobial activity of LysF1, since the lytic activity of the truncated variant containing the CHAP domain alone was decreased. The results of a co-sedimentation assay of SH3b domain showed that it was able to bind to three types of purified staphylococcal peptidoglycan 11.2, 11.3, and 11.8 that differ in their peptide bridge, but also to the peptidoglycan type 11.5 of Streptococcus uberis, and this capability was verified in vivo using the fusion protein with GFP and fluorescence microscopy. Using several different approaches, including NMR, we have not confirmed the previously proposed interaction of the SH3b domain with the pentaglycine bridge in the bacterial cell wall. The new naturally raised deletion mutant endolysin LysF1 is smaller than LysK, has a broad lytic spectrum, and therefore is an appropriate enzyme for practical use. The binding spectrum of SH3b domain covering all known staphylococcal peptidoglycan types is a promising feature for creating new chimeolysins by combining it with more effective catalytic domains.

Microtubule-associated proteins (MAPs) are abundantly present in axons and dendrites, and have been shown to play crucial role during the neuronal morphogenesis. The period of main dendritic outgrowth and synaptogenesis coincides with high expression levels of one of MAPs, the MAP2c, in rats. The MAP2c is a 49.2 kDa intrinsically disordered protein. To achieve an atomic resolution characterization of such a large protein, we have developed a protocol based on the acquisition of two five-dimensional (13)C-directly detected NMR experiments. Our previously published 5D CACONCACO experiment (Nováček et al. in J Biomol NMR 50(1):1-11, 2011) provides the sequential assignment of the backbone resonances, which is not interrupted by the presence of the proline residues in the amino acid sequence. A novel 5D HC(CC-TOCSY)CACON experiment facilitates the assignment of the aliphatic side chain resonances. To streamline the data analysis, we have developed a semi-automated procedure for signal assignments. The obtained data provides the first atomic resolution insight into the conformational state of MAP2c and constitutes a model for further functional studies of MAPs.

β‐Glucosidases such as Zm‐p60.1 (Zea mays) and Bgl4:1 (Brassica napus) have implicated roles in regulating plant development by releasing biologically active cytokinins from O‐glucosides. A key determinant of substrate specificity in Zm‐p60.1 is the F193–F200–W373–F461 cluster. However, despite sharing the same substrates, amino acids in the active sites of Zm‐p60.1 and Bgl4:1 differ dramatically. In members of the Brassicaceae we found a group of β‐glucosidases sharing both high similarity to Bgl4:1 and a consensus motif A‐K‐K‐L corresponding to the F193–F200–W373–F461 cluster. To study the mechanism of substrate specificity further, we generated and analyzed four single (F193A, F200K, W373K and F461L) and one quadruple (F193A–F200K–W373K–F461L) mutants of Zm‐p60.1. The F193A mutant showed a specific increase in affinity for a small polar aglycone, and a deep decrease in kcat compared with the wild‐type. Formation of a cavity with decreased hydrophobicity, and significant consequent alterations in ratios of reactive and non‐reactive complexes, revealed by computer modeling, may explain the observed changes in kinetic parameters of the F193 mutant. The large decrease in kcat for the W373K mutant was unexpected, but the findings are consistent with the F193–aglycone–W373 interaction playing a dual role in the enzyme’s catalytic action; influencing both substrate specificity, and the catalytic rate by fixing the glucosidic bond in a favorable orientation for attack by the catalytic pair. Investigation of the combined effects of all of the mutations in the quadruple mutant of Zm‐p60.1 was precluded by extensive alterations in its structure and almost complete abolition of its enzymatic activity.

Histidine-containing phosphotransfer proteins from Arabidopsis thaliana (AHP1-5) act as intermediates between sensor histidine kinases and response regulators in a signalling system called multi-step phosphorelay (MSP). AHP proteins mediate and potentially integrate various MSP-based signalling pathways (e.g. cytokinin or osmosensing). However, structural information about AHP proteins and their importance in MSP signalling is still lacking. To obtain a deeper insight into the structural basis of AHP-mediated signal transduction, the three-dimensional structure of AHP2 was determined. The AHP2 coding sequence was cloned into pRSET B expression vector, enabling production of AHP2 fused to an N-terminal His tag. AHP2 was expressed in soluble form in Escherichia coli strain BL21 (DE3) pLysS and then purified to homogeneity using metal chelate affinity chromatography and anion-exchange chromatography under reducing conditions. Successful crystallization in a buffer which was optimized for thermal stability yielded crystals that diffracted to 2.5 Å resolution.

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