Meltrin beta (ADAM19) mediates ectodomain shedding of Neuregulin beta1 in the Golgi apparatus: fluorescence correlation spectroscopic observation of the dynamics of ectodomain shedding in living cells.

Membrane-anchored Neuregulin beta1 sheds its ectodomain as soluble factors. Two proteases that belong to a disintegrin and metalloprotease (ADAM) family are known to cleave Neuregulin beta1. One is tumor necrosis factor-alpha converting enzyme (TACE/ADAM17). The other is Meltrin beta (ADAM19). Against our expectation that shedding by ADAM proteases occurs at the cell surface, here we found that Meltrin beta mediates the ectodomain shedding of Neuregulin beta1 in the Golgi apparatus. Meltrin beta was localized in and around the Golgi apparatus in developing sensory neurons. Subcellular fractionation revealed that Meltrin beta generated soluble Neuregulin beta1 in Golgi-enriched fractions while TACE-cleaved Neuregulin beta1 was recovered in lighter fractions. To examine whether Meltrin beta-mediated ectodomain shedding occurs in the Golgi apparatus in living cells, we took advantage of different diffusion properties of cleavage products from those of membrane-anchored precursor proteins. Fluorescence correlation spectroscopy (FCS) is the most sensitive method to determine milli approximately submillisecond diffusion in vivo. Protease-active Meltrin beta caused a shift in autocorrelation function in FCS of green fluorescent protein (GFP)-tagged Neuregulin beta1 in the Golgi apparatus, suggesting a conversion of Neuregulin beta1 molecules from membrane-anchored to soluble forms in that organelle. The Golgi apparatus is a site of processing Neuregulin beta1 by Meltrin beta.

An optimized homogeneous noncompetitive immunoassay based on the antigen-driven enzymatic complementation.

We describe an optimized noncompetitive and homogeneous immunoassay based on the antigen-dependent reassociation of antibody variable domains and beta-galactosidase (beta-gal) complementation (open sandwich enzymatic complementation immunoassay, OS-ECIA). The reassociation of two fusion proteins, an antibody heavy chain variable region fragment tethered to an N-terminal deletion mutant of beta-gal, V(H)Deltaalpha, and the light chain variable region fragment tethered to a C-terminal deletion mutant of beta-gal, V(L)Deltaomega, was monitored by the enzymatic complementation between the two. With the use of anti-hen egg lysozyme (HEL) antibody HyHEL10, an antigen-dependent enhancement in the enzymatic activity was clearly observed. To optimize the assay, the lengths of the linkers connecting the two domains of each fusion protein were varied, and the optimal pair V(H)(G(4)S)(2)Deltaalpha/V(L)(G(4)S)Deltaomega showed much improved antigen-responsive beta-gal activity. After various optimizations, almost 1000-fold improvement in sensitivity compared with that of our corresponding homogeneous open sandwich (OS) assays based on the energy transfer was observed, possibly due to lower V(H)/V(L) concentration and background heterodimer association.

A homogeneous noncompetitive immunoassay for the detection of small haptens.

We describe a noncompetitive homogeneous immunoassay for small haptens based on the antigen-dependent reassociation of antibody variable domains and beta-galactosidase (beta-gal) complementation (open sandwich enzymatic complementation immunoassay). As a model system, the reassociation of two fusion proteins, an anti 4-hydroxy-3-nitrophenylacetyl (NP) antibody heavy-chain variable-region fragment fused to an N-terminal deletion mutant of beta-gal (V(H)delta alpha) and the light-chain variable-region fragment fused to a C-terminal deletion mutant of beta-gal (V(L)delta omega), was monitored by the enzymatic complementation between the two. Upon simple mixing of the reagents with the sample, an antigen (NP)-dependent increase in enzymatic activity was observed. When 5-iodo-NP was measured, a 10 times higher sensitivity was observed, probably due to its higher affinity. Compared with our corresponding heterogeneous open sandwich enzyme-linked immunosorbent assay, approximately 1000-fold improvement in the sensitivity was attained, probably due to lower background V(H)-V(L) association. In addition, the assay required less time, handling, sample volume, and assay reagents.