基金This work was supported by grants from the National Natural Science Foundation of China (30370694, 30421005, 30623003, 30400245 and 30630036), the Ministry of Science and Technology of China (2002CB513006, 2006CB943902 and 2006AA02Z 169), the Chinese Acad- emy of Sciences (KSCX2-YW-R-67 and KJCX2-YW-H08) and the Shanghai Municipal Commission for Science and Technology (04JC14078, 06DZ22032, 055407035 and 058014578).