Maple syrup urine disease: further evidence that newborn screening may fail to identify variant forms

Newborn screening (NBS) by tandem mass spectrometry (MS/MS) has allowed for early detection and initiation of treatment in many patients with maple syrup urine disease (MSUD) (OMIM 248600), however, a recent report suggests that variants forms may be missed. Information on these patients is limited. We present clinical, biochemical and molecular information on patients with variant forms of MSUD not detected by the California Newborn Screening Program. Between July 2005 and July 2009, 2200,000 newborns were screened in California by MS/MS. Seventeen cases of MSUD were detected and three (two siblings) were missed. Additionally, the NBS cards of two siblings with late onset MSUD, who were born pre-expanded NBS, were retrospectively analyzed. None of the five patients met criteria to be considered presumptive positive for MSUD (leucine>200micromol/L and a ratio of leucine/alanine>or=1.5). Alloisoleucine (allo-ile) was subsequently analyzed in the NBS cards of all five patients, two of whom were found to have elevated levels. The proband in each family was diagnosed following symptoms triggered by an intercurrent illness or increased protein intake. At diagnosis, leucine levels ranged between 561 and >4528micromol/L, and allo-ile ranged from 137 to 239micromol/L. Two affected siblings had normal plasma amino acids when asymptomatic; however, their biochemical profiles were diagnostic of MSUD during intercurrent illnesses. The median age at diagnosis of all patients was one year (range 0.8-6.7). Heterozygous BCKDHB (E1beta) mutations (c.832G>A/c.970C>T) were identified in one family and a homozygous DBT (E2) sequence variant (c.1430 T>G) in another. The third family had one identifiable DBT mutation (c.827T>G), however, a second mutation was not detected. This report provides further evidence that NBS by MS/MS is unable to detect all cases of MSUD. Second-tier testing with allo-ile may improve sensitivity; however, some children with variant forms will invariably be missed.

Prediction and confirmation of a site critical for effector regulation of RGS domain activity

A critical challenge of structural genomics is to extract functional information from protein structures. We present an example of how this may be accomplished using the Evolutionary Trace (ET) method in the context of the regulators of G protein signaling (RGS) family. We have previously applied ET to the RGS family and identified a novel, evolutionarily privileged site on the RGS domain as important for regulating RGS activity. Here we confirm through targeted mutagenesis of RGS7 that these ET-identified residues are critical for RGS domain regulation and are likely to function as global determinants of RGS function. We also discuss how the recent structure of the complex of RGS9, Gt/i1alpha-GDP-AlF4- and the effector subunit PDEgamma confirms their contact with the effector-G protein interface, forming a structural pathway that communicates from the effector-contacting surface of the G protein and RGS catalytic core domain to the catalytic interface between Galpha and RGS. These results demonstrate the effectiveness of ET for identifying binding sites and efficiently focusing mutational studies on their key residues, thereby linking raw sequence and structure data to functional information.

A regulator of G protein signaling interaction surface linked to effector specificity

Proteins of the regulator of G protein signaling (RGS) family accelerate GTP hydrolysis by the alpha subunits (G(alpha)) of G proteins, leading to rapid recovery of signaling cascades. Many different RGS proteins can accelerate GTP hydrolysis by an individual G(alpha), and GTP hydrolysis rates of different G(alpha)s can be enhanced by the same RGS protein. Consequently, the mechanisms for specificity in RGS regulation and the residues involved remain unclear. Using the evolutionary trace (ET) method, we have identified a cluster of residues in the RGS domain that includes the RGS-G(alpha) binding interface and extends to include additional functionally important residues on the surface. One of these is within helix alpha3, two are in alpha5, and three are in the loop connecting alpha5 and alpha6. A cluster of surface residues on G(alpha) previously identified by ET, and composed predominantly of residues from the switch III region and helix alpha3, is spatially contiguous with the ET-identified residues in the RGS domain. This cluster includes residues proposed to interact with the gamma subunit of G(talpha)'s effector, cGMP phosphodiesterase (PDEgamma). The proximity of these clusters suggests that they form part of an interface between the effector and the RGS-G(alpha) complex. Sequence variations in these residues correlate with PDEgamma effects on GTPase acceleration. Because ET identifies residues important for all members of a protein family, these residues likely form a general site for regulation of G protein-coupled signaling cascades, possibly by means of effector interactions.

Formation of helical protein assemblies of IgG and transducin on varied lipid tubules

Helical protein arrays on lipid tubules are valuable assemblies for studying protein structure and protein-lipid interactions through electron microscopy and crystallography. We describe conditions for forming such arrays from two proteins, IgG and transducin, the photoreceptor G protein, using a variety of lipid surfaces. Anti-dinitrophenyl (DNP) IgG arrays formed on DNP-phosphatidylethanolamine (DNP-PE) mixed with either galactosyl-ceramide lipids or phosphatidylcholine (PC) display different pH sensitivities and dimensions, yet have similar helical symmetries. DNP-PE/PC mixtures formed small crystals and large well-ordered flattened tubules. The peripheral membrane protein transducin (G(t)) formed helical arrays either on a mixture of cationic and neutral lipids or on residual photoreceptor lipids. Despite differences in lipid composition, the G(t) arrays have similar structures and show similar sensitivity to activation and variations in ionic environment. G(t) activation causes the helical assemblies to collapse to small vesicles, a process resembling the vesiculation of activated dynamin-lipid tubules. In a preliminary three-dimensional reconstruction, the hapten-bound IgG appears to make two contacts to the central lipid tubule, presumably via the F(ab) domains. The ability to generate a three-dimensional reconstruction without tilts illustrates one advantage of helical structures for two-dimensional crystallography, especially for visualizing protein-lipid interactions.