Regulatory heme binds to specific motifs in proteins and controls a variety of biochemical processes. Several of these proteins were recently shown to form complexes with ferric and/or ferrous heme via a cysteine residue as axial ligand. The objective of this study was to examine the heme-binding properties of a series of cysteine-containing peptides with focus on CP motif sequences. The peptides displayed different binding behavior upon Fe(III) heme application with characteristic wavelength shifts of the Soret band to 370 nm or 420-430 nm and in some cases to both wavelengths. Whereas for most of the peptides containing a cysteine only a shift to 420-430 nm was observed, CP-containing peptides exhibited a preference for a shift to 370 nm. Detailed structural investigation using Raman and NMR spectroscopy on selected representatives revealed different binding modes with respect to iron ion coordination, which reflected the results of the UV-vis studies. A predicted short sequence stretch derived from dipeptidyl peptidase 8 was additionally examined with respect to CP motif binding to heme on the peptide as well as on the protein level. The heme association was confirmed with the first solution structure of a CP-peptide-heme complex and, moreover, an inhibitory effect of Fe(III) heme on the enzyme's activity. The relevance of both the use of model compounds to elucidate the molecular mechanism underlying regulatory heme binding and its potential for the investigation of regulatory heme control is discussed.

The integrity of the plasma membrane is maintained through an active repair process, especially for skeletal and cardiac muscle cells, in which contraction-induced mechanical damage frequently occurs in vivo1,2. Muscular dystrophies (MDs) are a group of muscle diseases characterized by skeletal muscle wasting and weakness3,4. An important cause of MD is defective repair of sarcolemmal injuries, and sarcolemma repair requires Ca2+ sensor proteins5–8 and Ca2+-dependent delivery of intracellular vesicles to injury sites5,8,9. TRPML1 (ML1) is an endosomal and lysosomal Ca2+ channel and its human mutations cause Mucolipidosis IV, a neurodegenerative disease with motor disabilities10,11. Here, we report that ML1-null mice develop a primary, early-onset muscular dystrophy independent of neural degeneration. Although the dystrophin-glycoprotein complex and the known membrane repair proteins are normally expressed, membrane resealing was defective in ML1-null muscle fibers or upon acute and pharmacological inhibition of ML1 channel activity or vesicular Ca2+ release. Injury facilitated the trafficking and exocytosis of vesicles by upmodulating ML1 channel activity. In the dystrophic mdx mouse model, overexpression of ML1 decreased muscle pathology. Collectively, we have identified an intracellular Ca2+ channel that regulates membrane repair in skeletal muscle via Ca2+-dependent vesicle exocytosis.

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Ion-dependent channels and transporters have been identified in lysosomes, including the V-ATPase H+ pump and transient receptor potential mucolipin channels (TRPMLs), the principle Ca2+ release channels in the lysosome, but much less is understood about the roles of Na+ and K+ in lysosomal physiology. Wang et al. describe a voltage-sensitive, Ca2+-activated K+ current in the lysosome (LysoKVCa) and show that LysoKVCa regulates lysosomal membrane potential and refilling of lysosomal Ca2+ stores.