SEC31:SEC13 and v-SNARE recruitment

The outer coat proteins SEC13 and SEC31A are thought to exist in a cytosolic heterohexamer with SEC23IP and are recruited to the ER through interaction with pre-bound SEC23-SEC24 complexes (Stephens et al, 2000; Hammond and Glick, 2000; Stagg et al. 2006; Ong et al, 2010; reviewed in Szul and Sztul, 2011). Human SEC31B is close to 50% identical to human SEC31A (Tang et al. 2000, Stankewich et al. 2006). While SEC31A is ubiquitously and abundantly expressed, SEC31B is expressed at a high level in testis (Tang et al. 2000: Northern blot; Stankewich et al. 2006: Northern blot and dot-blot), thymus (Tang et al. 2000: Northern blot; Stankewich et al. 2006: dot-blot), cerebellum (Stankewich et al. 2006: dot-blot), lymph node (Stankewich et al. 2006: dot-blot), and pituitary gland (Stankewich et al. 2006: dot-blot) and at a low level in other tissues (Tang et al. 2000; Stankewich et al. 2006). SEC31B can form dimers with SEC31A (Stankewich et al. 2006) and it co-immunoprecipitates and co-localizes with at least two components of the COPII coat, SEC23 and SEC13 (Stankewich et al. 2006). The dynamic behavior of SEC31B in the cells is consistent with its participation with COPII-coated organelles (Stankewich et al. 2006). In a zebrafish model of retinal development, oligonucleotide-mediated gene knockdown of either sec31a or sec31b leads to loss of COPII function (Niu et al. 2014). In Arabidopsis, SEC31A and SEC31B were both shown to function as COPII components that are essential and interchangeable in pollen development (Liu et al. 2021). Based on experimental evidence described above, SEC31B is annotated as a candidate SEC31 component of the COPII coat. v-SNAREs such as the SEC22 proteins are also incorporated into the emerging vesicle through interactions with components of the COPII coat (Xu et al, 2000; Mancias and Goldberg, 2008; Gordon et al, 2010; reviewed in Szul and Sztul, 2011; Lord et al, 2013).

外层衣壳蛋白SEC13和SEC31A被认为与SEC23IP存在于细胞质中的异六聚体中，并通过与预先结合的SEC23-SEC24复合物的相互作用被招募到内质网（Stephens等，2000；Hammond和Glick，2000；Stagg等，2006；Ong等，2010；参见Szul和Sztul，2011的综述）。人源SEC31B与SEC31A的相似度高达50%（Tang等，2000，Stankewich等，2006）。尽管SEC31A在体内广泛且大量表达，SEC31B在睾丸（Tang等，2000：Northern blot；Stankewich等，2006：Northern blot和dot-blot）、胸腺（Tang等，2000：Northern blot；Stankewich等，2006：dot-blot）、小脑（Stankewich等，2006：dot-blot）、淋巴结（Stankewich等，2006：dot-blot）和垂体腺（Stankewich等，2006：dot-blot）中高表达，而在其他组织中低表达（Tang等，2000；Stankewich等，2006）。SEC31B可以与SEC31A形成二聚体（Stankewich等，2006），并与至少两种COPII衣壳成分SEC23和SEC13共免疫沉淀和共定位（Stankewich等，2006）。SEC31B在细胞中的动态行为与其参与COPII包裹的细胞器相一致（Stankewich等，2006）。在斑马鱼视网膜发育模型中，通过寡核苷酸介导的sec31a或sec31b基因敲低导致COPII功能的丧失（Niu等，2014）。在拟南芥中，SEC31A和SEC31B均被证实作为COPII成分在花粉发育中发挥不可或缺且可互换的作用（Liu等，2021）。基于上述实验证据，SEC31B被标注为COPII衣壳的SEC31候选成分。v-SNAREs，如SEC22蛋白，也通过与COPII衣壳成分的相互作用被纳入新兴的囊泡中（Xu等，2000；Mancias和Goldberg，2008；Gordon等，2010；参见Szul和Sztul，2011；Lord等，2013的综述）。