Transport of storage proteins to the vacuole is mediated by vesicles without a clathrin coat

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Abstract
Storage parenchyma cells of developing legume cotyledons actively transport large amounts of storage proteins to protein storage vacuoles (PSV). These proteins are synthesized on the endoplasmic reticulum and pass through the Golgi apparatus. Clathrin coated vesicles (CCV) and small electron dense vesicles found near the trans-Golgi network (TGN) have both been implicated in the Golgi-to-vacuole transport step. Recent findings that protein storage cells contain more than one type of vacuole have necessitated a re-examination of the role of both types of vesicles in vacuolar protein transport. Immunoblots of highly purified CCV preparations and immunogold labelling with antibodies to the storage proteins vicilin and legumin, indicate that the dense vesicles, but not the CCV, are involved in storage protein transport in pea cotyledons. This result is supported by the finding that α-TIP, a protein characteristic of the PSV membrane, is absent from CCV. In addition, complex glycoproteins appear to be carried by CCV but are not detectable in the PSV. We suggest on the basis of these data that storage proteins and other vacuolar proteins such as acid hydrolases are not sorted by the same mechanism and are transported by different types of vesicles to different types of vacuoles.