To maintain protein homeostasis in secretory compartments, eukaryotic cells harbor a quality control system that monitors protein folding and protein complex assembly in the endoplasmic reticulum (ER). Proteins that do not fold properly or integrate into cognate complexes are degraded by ER-associated degradation (ERAD) involving retrotranslocation to the cytoplasm and proteasomal peptide hydrolysis. N-linked glycans are essential in glycoprotein ERAD; the covalent oligosaccharide structure is used as a signal to display the folding status of the host protein. In this study, we define the function of the Htm1 protein as an α1,2-specific exomannosidase that generates the Man7GlcNAc2 oligosaccharide with a terminal α1,6-linked mannosyl residue on degradation substrates. This oligosaccharide signal is decoded by the ER-localized lectin Yos9p that in conjunction with Hrd3p triggers the ubiquitin-proteasome–dependent hydrolysis of these glycoproteins. The Htm1p exomannosidase activity requires processing of the N-glycan by glucosidase I, glucosidase II, and mannosidase I, resulting in a sequential order of specific N-glycan structures that reflect the folding status of the glycoprotein.

Nascent and newly synthesized glycoproteins enter the calnexin (Cnx)/calreticulin (Crt) cycle when two out of three glucoses in the core N-linked glycans have been trimmed sequentially by endoplasmic reticulum (ER) glucosidases I (GI) and II (GII). By analyzing arrested glycopeptides in microsomes, we found that GI removed the outermost glucose immediately after glycan addition. However, although GII associated with singly glycosylated nascent chains, trimming of the second glucose only occurred efficiently when a second glycan was present in the chain. Consistent with a requirement for multiple glycans to activate GII, pancreatic RNase in live cells needed more than one glycan to enter the Cnx/Crt cycle. Thus, whereas GI trimming occurs as an automatic extension of glycosylation, trimming by GII is a regulated process. By adjusting the number and location of glycans, glycoproteins can instruct the cell to engage them in an individually determined folding and quality control pathway.

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ColQ, the collagen tail subunit of asymmetric acetylcholinesterase, is responsible for anchoring the enzyme at the vertebrate synaptic basal lamina by interacting with heparan sulfate proteoglycans. To get insights about this function, the interaction of ColQ with heparin was analyzed. For this, heparin affinity chromatography of the complete oligomeric enzyme carrying different mutations in ColQ was performed. Results demonstrate that only the two predicted heparin-binding domains present in the collagen domain of ColQ are responsible for heparin interaction. Despite their similarity in basic charge distribution, each heparin-binding domain had different affinity for heparin. This difference is not solely determined by the number or nature of the basic residues conforming each site, but rather depends critically on local structural features of the triple helix, which can be influenced even by distant regions within ColQ. Thus, ColQ possesses two heparinbinding domains with different properties that may have non-redundant functions. We hypothesize that these binding sites coordinate acetylcholinesterase positioning within the organized architecture of the neuromuscular junction basal lamina.At vertebrate cholinergic synapses, acetylcholinesterase (AChE) 1 rapidly hydrolyzes the neurotransmitter acetylcholine, thereby terminating synaptic transmission. This key function does not only require a high catalytic turnover number but also a strategic positioning of the enzyme. This is achieved by the association of AChE catalytic subunits with structural subunits that bring them to the site of action. In the case of asymmetric AChE, the collagen ColQ is responsible for the localization of the enzyme at the vertebrate neuromuscular junction. Inactivation of the ColQ gene in mice or mutations in the human ColQ gene result in the absence of enzyme accumulation at the neuromuscular junction and are the cause of a congenital myasthenic syndrome (type 1c) (1, 2).As found in other collagens, ColQ contains a central triplehelical domain surrounded by non-collagenous N-and C-terminal domains (Fig. 1A). Each N-terminal domain organizes an AChE tetramer, so the triple-helical structure generates an A 12 or asymmetric AChE form with 12 catalytic subunits (3, 4). The collagen domain is characterized by Gly-Xaa-Yaa repeats and a high proportion of the stabilizing proline and hydroxyproline residues. The C-terminal domain is divided into a proline-rich region, important for triple-helix formation (5), and a cysteinerich region probably involved in the anchorage of AChE at the neuromuscular junction, since mutations in this region prevent the accumulation of AChE in congenital myasthenic syndrome type 1c patients (2).Heparan sulfate proteoglycans (HSPGs) have been implicated in the anchorage of asymmetric AChE to the synaptic basal lamina by interacting with ColQ through their heparan sulfate (HS) chains. This was proposed after showing that AChE could be specifically solubilized from the tissue with heparinase as well as with HS and ...