a='0.424870551E+03-0.100113019E+03 0.470220758E+03';

b=str2num(a);

b =324.7575  470.2208;

>> sscanf(a,'%E')
ans =
  424.8706
 -100.1130
  470.2208

a = '0.424870551E+03-0.100113019E+03 0.470220758E+03';
ind = a=='-'; %// detect positions of "-" ...
ind2 = [0 a(1:end-1)=='E'];
ind = ind & ~ind2; %// ... but not in exponents. Thanks to Rafael Monteiro
asep = repmat(' ',1,numel(a)+nnz(ind)); %// initiallize with spaces
asep((1:numel(a))+cumsum(ind)) = a; %// fill in a. Leave a blank before each "-"
b = str2num(asep);

a =
0.424870551E+03-0.100113019E+03 0.470220758E+03

asep =
0.424870551E+03 -0.100113019E+03 0.470220758E+03

b =
  424.8706 -100.1130  470.2208

str2num('0.424870551E+03-0.100113019E+03 0.470220758E+03')
str2num('0.424870551E+03 - 0.100113019E+03 0.470220758E+03')
str2num('0.424870551E+03- 0.100113019E+03 0.470220758E+03')

str2num('0.424870551E+03 -0.100113019E+03 0.470220758E+03')