Biology Reference
In-Depth Information
Sample uniformity with non-crop related, long-lived seeds,
such as those of trees, can be especially problematic. While seeds
collected from mature forests are potentially of clonal origin, it is
probably best to assume that they are not. In such cases (e.g.,
40
,
41
), the only real option is to describe as thoroughly as practical
where/how the seeds were collected in the wild, and either main-
tain or deposit a sample with some sort of botanical archiving ser-
vice. Finally, it is best to remain conservative in interpretation of
the results of proteomic analyses especially in regard to low abun-
dance proteins.
3
Storage Proteins
3.1 Terminology
The systematic terminology used to defi ne classes of SSP is
solubility-based [
42
]. Thus, the SSP soluble in H
2
O are called
albumins, while those soluble in dilute saline, aqueous alcohol, or
dilute alkali or acid are globulins, prolamins, or glutelins, respec-
tively. While albumins are found in all seeds, prolamins and glute-
lins are abundant in monocot seeds and globulins are prevalent in
dicot seeds [
43
,
44
]. The globulin SSP has additionally been
grouped based upon sedimentation into 7S or 11S [
45
].
Furthermore, there are a plethora of trivial names assigned to SSP
based on their Latin generic names: the zeins from maize (
Zea
mays
), hordeins from barley (
Hordeum vulgare
), secalins from rye
(
Secale cereale
), etc.
The SSP are synthesized as precursors on the rough-endoplasmic
reticulum (ER), targeted to the ER lumen by an N-terminal signal
sequence which is co-translocationally removed by signal peptidase
[
3
]. The SSP are then sorted from bulk protein traffi c through the
secretory pathway to their site of deposition, the protein storage
vacuole (PSV). There are two targeting pathways for SSP, one for
prolamins and a second for the non-prolamin SSP. The prolamin-
containing PSV arise by direct vesiculation from the ER; prolamins
do not traverse the classical secretory pathway. Non-prolamin SSP
passes through the classical secretory pathway, and is sorted from a
post-Golgi compartment to the PSV [
46
].
The prolamin PSV can have a complex internal architecture,
which is thought to be the result of different rates of synthesis of
the various subunits [
47
]. In contrast, non-prolamin PSV has a
uniformly granular appearance [
48
]. An additional morphological
characteristic of prolamin-PSV is, since they are directly derived
from the rough-ER, that they are studded with ribosomes [
49
].
Rice (
Oryza sativa
) endosperm is unusual in containing both pro-
lamin- and non-prolamin PSV [
50
].
The 2S, 7S, and 11S SSP are synthesized as large precursors
(now known to be prepro-polyproteins) [
3
] which undergo
3.2 Synthesis
and Processing
