Stromal membranes

Freeze-fracture electron microscopy of thylakoid membranes has clearly revealed an asymmetric lateral distribution of the various photosynthetic complexes in the granal and stromal membranes, i.e., the distribution of the protein complexes in the membrane is nonrandom. This lateral asymmetry was further substantiated by the results of electron microscopy of the inside-out vesicles discussed in Section Vll. These findings by electron microscopy are summarized by the model shown in Fig. 21 (A). It is a transverse cross section of the thylakoids shown earlier in Fig. 13 (D) and (D ), with the various photosynthetic protein complexes appropriately placed in the granal and stromal regions. 

Four separate pathways for transporting proteins from the stroma into the thylakoid have been identified. All four pathways have been found to be closely related to analogous transport mechanisms in bacteria, illustrating the close evolutionary relationship between the stromal membrane and the bacterial inner membrane. Transport of plastocyanin and related proteins into the thylakoid lumen occurs by an SRP-dependent pathway (Figure 16-31, path A). A second pathway for transporting proteins into the thylakoid lumen... 

Shielding of Cytochrome b at the Stromal Surface. CpA has no effect on cyt b (9), an expected result since the Pro residue at the penultimate position should block CpA (10). However, the efficacy of trypsin digestion was increased if the membranes were treated first with CpA. o-phenanthroline was added to inhibit CpA before trypsin treatment so that the two proteases did not act simultaneously. Reversing the order of protease addition had no effect, showing that the access of trypsin was increased by prior treatment with CpA. Similar results were obtained using antibodies (i) and (iii) to the NHj- and COOH-termini (unpubl.). Thus, cyt b on the stromal membrane surface is shielded by the COOH-termini of other thylakoid proteins. 

Lane 1 molecular weight standards, lanes 2, 11 dark adapted stromal and granal membranes kept at 15 C lanes 16, 18 granal and stromal membranes after 7 hr in the light at 25 C lanes 19, 22 stromal and granal membranes after 7 hr in the light at 3 C. 

FIGURE 22.20 The molecular architecture of PSI. PsaA and PsaB constitute the reaction center dimer, an integral membrane complex P700 is located at the lumenal side of this dimer. PsaC, which bears Fe-S centers and Fb, and PsaD, the interaction site for ferre-doxin, are on the stromal side of the thylakoid membrane. PsaF, which provides the plasto-cyaiiin interaction site, is on the lumenal side. (Adapted from Golbeck, J. H., 1992. Amiual Review of Plant Physiology and. Plant Molecular Biology 43 293-324.)... 

Little is known of how the biosynthetic metabolon is assembled, what mechanisms control the membrane-specific targeting, and how the conversions to apocarotenoids occur. Yet the current approach to drive import of bacterial or plant genes is to use transit sequences of a stromal protein that may limit the effectiveness of the transgene. In addition, for specific applications of controlling carotenoid composition, we need to better understand the interactions of the various enzymes,... 

Basement membrane proteins Platelet activating factors Stromal proteins... 

Extended biological investigations concerning structure-function studies were further initiated to evaluate the abilities of these clusters to inhibit Con A-induced membrane type-1-matrix metalloproteinase (MTl-MMP)-mediated pro-MMP-2 activation, cell death, and antiproliferative property in mesenchymal stromal cells (MSC).83... 

Another 3-D cornea model, comprising rabbit primary cultures of epithelial and stromal cells as well as mouse immortalized endothelial cells, was described in 1994 by Zieske and coworkers [70], They showed the influence of endothelial cells on the formation of a tightly packed, multilayered epithelium as well as the expression of laminin, type VII collagen, a6 integrin, keratin K3, and a-enolase. Furthermore, their findings suggested that the formation of an in vivo-like epithelium requires the cultivation of the 3-D corneal construct under AIC conditions. By contrast, LCC methods of cultivating corneal equivalents in the absence of endothelial cells failed to promote the expression of differentiation markers and basement membrane components. 

To verify the different functions of the isoforms of SCF and CSF-1 in promoting stroma-dependent growth a non-stromal cell system was used to express the soluble and membrane-bound cDNA isoforms. 

---